I am running Icewind Dale: Rime of the Frostmaiden currently. By the end, the PCs will probably be about 12th level (due to extra stuff I'm adding). Afterwards, I want to run Baldur's Gate: Descent Into Avernus, continuing with the same characters/story/canon. I came here looking for tips on how to run Baldur's Gate: Descent Into Avernus at such a high level. Any advice? Thank you in advance.

I just scale the mobs up and change the lore a bit. No reason a goblin can't be a cr12 group based on new lore. That's the easiest way. Least that's all i do to keep it simple. 

Give those old weak mobs new skills and spells to throw the party off. Homebrew them all. 

This thread isn't based around 12th level characters, but it could help: https://www.dndbeyond.com/posts/599-road-to-baldurs-gate-wrap-up-avernus-or-bust

I'd skip the first chapter, set in Baldur's Gate.  Since your characters have established themselves as heroes up in Icewind Dale, have them summoned by the Lords Alliance or similar faction who have figured out the infernal plot behind the disappearance of Elturel and the for now frustrated effort to bring Baldur's Gate with it.  I'm going with Lords Alliance because while different members may disagree on their disposition toward Baldur's Gate, the chaos brought to the city plus the security vacuum left by the disappearance of Elturel (perhaps pious to a fault but did good for the region in suppressing undead and fiendish activity in the outlying regions) brings the regional governments some concern.  So, take this holiphant and off to Hell heroes!

Really the first chapter plays like a D&D primer, and really unless you as the DM worked up a backstory tie to Elturel, the PCs who vanquish the Vanthampurs don't have a lot of reason to go to Avernus aside "it's the right thing to do."  The continuity between chapter one and the hell chapters (aside from Ulder Ravengaard, but he's one of the reason's your going) is basically Thavius Krieg who is sort of presented as an optional encounter and not a particularly powerful one.

Another hook.  If I remember correctly, a lot of what's going on in Icewind Dale during Rime of the Frostmaiden is partially due to an infernal plot.  If the heroes were successful, they already have an enemy in the Hells, and that enemy could be a force multiplier to the hellish and demonic challenges already written in the DitA.

Once in Hell, I've fleshed out some things and really ran with one of the NPC entity's background down there to give the players an opportunity to involve themselves in a sort of "great game" going on Hell, particularly Avernus, beyond the Elturel crisis.  Happy to share more over DM since I don't want to throw all my cards and dominate this thread.  But basically Hell's a sandbox that can become a fun quagmire that can take your party a number of different directions while still on or abandoning the mission.

Quote from MidnightPlat >>

I'd skip the first chapter, set in Baldur's Gate.  Since your characters have established themselves as heroes up in Icewind Dale, have them summoned by the Lords Alliance or similar faction who have figured out the infernal plot behind the disappearance of Elturel and the for now frustrated effort to bring Baldur's Gate with it.  I'm going with Lords Alliance because while different members may disagree on their disposition toward Baldur's Gate, the chaos brought to the city plus the security vacuum left by the disappearance of Elturel (perhaps pious to a fault but did good for the region in suppressing undead and fiendish activity in the outlying regions) brings the regional governments some concern.  So, take this holiphant and off to Hell heroes!

Really the first chapter plays like a D&D primer, and really unless you as the DM worked up a backstory tie to Elturel, the PCs who vanquish the Vanthampurs don't have a lot of reason to go to Avernus aside "it's the right thing to do."  The continuity between chapter one and the hell chapters (aside from Ulder Ravengaard, but he's one of the reason's your going) is basically Thavius Krieg who is sort of presented as an optional encounter and not a particularly powerful one.

Another hook.  If I remember correctly, a lot of what's going on in Icewind Dale during Rime of the Frostmaiden is partially due to an infernal plot.  If the heroes were successful, they already have an enemy in the Hells, and that enemy could be a force multiplier to the hellish and demonic challenges already written in the DitA.

Once in Hell, I've fleshed out some things and really ran with one of the NPC entity's background down there to give the players an opportunity to involve themselves in a sort of "great game" going on Hell, particularly Avernus, beyond the Elturel crisis.  Happy to share more over DM since I don't want to throw all my cards and dominate this thread.  But basically Hell's a sandbox that can become a fun quagmire that can take your party a number of different directions while still on or abandoning the mission.

 I would really appreciate if you have more, if you have the time. Thank all of you so much, this was all very helpful!

Happy to, I'll see if I can get some notes up here on my next prep session.  In the meantime, for inspiration you might want to check out this thread.  It started with he OP wanting feed back for how he was structuring the forces of hell as military units.  In my feedback I went a bit broader.  Basically I see hell as a police state so am throwing in what I know of intelligence communities and state security services into the mix.  

I would really encourage you to keep whoever the Archdevil that was behind that plot in Rime of the Frostmaiden in play.  The PCs may not know that devil was involved, but it's likely the archdevil knows about the PCs.  The PCs thwarted a land grab basically and now are in the Hells to reclaim a city taken by Zariel.  However this devils regards Zariel (seems the arch devils are divided on her) your more chill infernal nemesis may work against the PCs out of pure vanity and spite, or maybe a larger scheme is put into place when the devil realizes the PCs are after the sword.

Levistus, that's the archdevil. And if he's working against the PCs, Geryon may offer some bargains to keep Levistus in check.  Or Levistus offers a bargain in the PCs hour of need, the PCs not realizing Levistus has a vindictive motive.

Ok wow thank you so much, that's all really helpful! I really appreciate this!

I did something similar earlier this year. I ran Descent into Avernus for 2 PCs after all of the Icepire Peak adventures, and the PCs were lv 14. For balancing combat, I mostly just increased the number of enemies or their strength across the board. If the combat encounter already had a lot of enemies, I typically swapped out some weaker fiends for more powerful ones with similar abilities, and if the encounters were with just a couple strong monsters, I often doubled the number of them. I did some math to make sure that relative difficulty of the fight stayed similar.

Story-wise, I kept almost everything in Avernus the same, but heavily changed up the beginning part of the adventure to fit the existing story of the PCs. My PCs cared about the town of Leilon, and had never ventured to Baldur's Gate, so I decided that Zariel's target would be Leilon, not Elturel. I changed the plot to be that the fiendish cults in the early part of the adventure were directed by Zariel to steal the Ruinstone, and use it to shift the town of Leilon into Avernus. I changed the lore around the Solar Insidiator to be just an angel that was enslaved, and its power was being siphoned by its cage to keep the town trapped in Avernus. This way, I could run the rest of the adventure mostly as written, only upscaling the combats and changing some dialog here and there so that Leilon fit in the place of Elturel, more or less.