I'm trying to create a homebrew barbarian centered around unarmed damage. At 3rd level, unarmed damage is supposed to be 1d6+Strength Modifier, I have that working just fine. I set several class features of the same name to replace this at their respective levels (1d8 at level 6, 1d10 at level 10, and 1d12 at level 14) but I can't seem to get them to take effect, the damage die just stays at 1d6.

Two things:

1. Replacement class features don’t work like that. They are for alternate subclass features to replace base subclass features with something different like the way some of the Optional Class Features from Tasha’s Cauldron work. You would need to make them as Granted features to get them to automatically supplant the earlier subclass feature of the same name.
2. You don’t need to do any of that at all. You can delete all of those extraneous subclass features, then go to the original subclass feature of the same name and edit it. There you would open the Action you created to put the attack on the character sheet, and edit that Action. Scroll all the way to the bottom and you will see a subsection for Higher Level Scaling. You can put those scaling dice values in there and the system will automatically update the attack whenever the character hits one of those levels.

I don't have it as an action though, I have it as a modifier replacing the character's base unarmed damage, and for some reason there's no option to scale that at higher levels.

And that worked?!? That hasn’t worked in the past, it wasn’t possible to replace unarmed strike damage with a modifier like that, a separate Action had to be created to add a new unarmed strike to the character sheet. Can you please confirm that your Modifier worked? Please post a link to your character sheet so I can see for myself. I’d greatly appreciate it.

yeah, under modifier type i chose "set" and then under modifier subtype chose "unarmed damage die" and was able to change it.. i got the scaling to work by making the upgraded features granted, but now i have the problem of several copies of the same subclass feature looking ugly in the character builder and the details page. wondering if i should just deal with it since i'm just using it for me and my players anyway.

link to character.

Okay, that was not an available Modifier Subtype before. And if what you say is true, then the glitch that let a newer subclass feature replace an older subclass feature of the same name has been fixed and that won’t work anymore either. (If that’s true then I’ll have to update the FAQ. Balls.)

If it were me I would drop the Modifier entirely and just create that Unarmed Strike as a new Action. By creating it as its own Action, you can then do the level scaling all internally and save yourself the headache. Yes, the original Unarmed Strike will still be visible on the character sheet, but that can be modified by simply customizing it and unchecking the “Display As Attack” box (or whatever it’s labeled as) so it no longer shows up under Actions. Or just leave it and have the ability to pull your punches with a normal Unarmed Strike.

PS- You can simplify the snippet text for that feature so it’s more concise and more informative on the character sheet if you wish. Just utilize the “Level Scaling” form attached to that subclass feature to have the appropriate dice count and die size at the correct levels, then replace the snippet with the following:

Your unarmed strike does {{scalevalue}}{{modifier:str}} bludgeoning damage, and an additional unarmed strike can be made as a bonus action.

sounds like my best bet, thanks for the advice!

Okay. I just ran a test and named two granted features on the same subclass with the same name, one at 3rd level and one at 7th. When I tested it on a character sheet the 7th level feature replaced the 3rd level feature under the Features & Traits section of the sheet, the 3rd level feature is nowhere to be seen.

Oh, wait, I get it now, I must have misread your post previously when you wrote “several copies of the same subclass feature looking ugly in the character builder and the details page.” You could avoid the ugliness in the builder by checking the “Hide in Builder” box for the subclass feature. Unfortunately there is no “Hide on Details Page” for subclass features like there is for racial traits, so they would still display there. So your best bet is still probably the Action instead of the Modifier. But if you don’t mind the details page having the additional instances of the feature you could do it as the Modifier. You could even tell the system to display all the additional instances of that feature at the very end of the subclass so that it wouldn’t look ugly until the very bottom of the page. Just something to consider.