So we use the entire stat block for the new form except for the hit points now?  Why is this better?

You get temporary hit points equal to the hit points of the new form.  Once those are gone you change back.  I'm not sure why the change.  Maybe so you can't heal your shape and stay in it longer?

This is OP infinite HP. when you shape-shift doesn't that when you cast the spell. so you can shape-shift as an action as a new beast. turn from a bloodied pit fiend into a full temp hp brass dragon and you're temp hp is restored way different than old shapechange

Hopefully they will adjust this so you only get temp HP from your first transformation. It has to be what they meant. I refuse to believe the devs for this game intentionally allowed level 17-20 characters to be able to just pick up hundreds of temp HP as an action on top of every other benefit the transformation gives you.

Can you use legendary actions while shapechanged?

Almost definitely not, as in 2014. But you should be able to use legendary resistance.

Quote from Sketching101 >>

Almost definitely not, as in 2014. But you should be able to use legendary resistance.

The 2014 spell's text read: "You can't use any legendary actions or lair actions of the new form."

Your character is getting the creature's stat-block with no limitations attached as to how that stat-block is used, so everything is on the table.

I am guessing that the 2025 MM won't have legendary actions, just multiple reactions (see Vecna for an example) and lair actions are technically outside of the stat block.

https://www.dndbeyond.com/monsters/2590978-vecna-the-archlich

not really infinite HP, you still have to hold concentration, and that's more likely to drop before you run out of HP on your given form anyway.

Moon druids get a bonus to concentration. and if you get war caster you have advantage. Working on your general con too. you could keep up con pretty easily.

Quote from Captivecreepa >>

Moon druids get a bonus to concentration. and if you get war caster you have advantage. Working on your general con too. you could keep up con pretty easily.

 but only while in wild shape

If they are shapechanged they aren't wild shaped

Does this mean when you shape shift you lose all your class features and feats until you revert back because it doesn’t specify you retain them but does specify the spellcasting feature? Also does Pact Magic count as the spellcasting for the purposes of this spell?

I like the old one it has more details and the other one is balanced 

Ok so reading the new MM, it looks like you get legendary resistance and legendary actions 

correct, this now the most broken spell in DnD

Adult metallic dragons can use this to turn into archmages, and other humanoids with spell casting now?

Quote from dandelion_msla >>

Adult metallic dragons can use this to turn into archmages, and other humanoids with spell casting now?

Not exactly. It says you retain your Spellcasting feature if you have it; since adult dragons have a Spellcasting feature, they retain it. This means that they can transform into an  Archmage but they still keep the same spell list they had as a dragon.

The spell says nothing about retaining feats, so you wouldn't have war caster or advantage.

fair, but like dragons and stuff have super high concentration and legendary resistances

Sure, but at level 17+ most things you're fighting will also hit like a truck. Even +11 will lose out to DC 30 more often than not. Legendary resistance I expect to be errata'd because that just feels like an unintentional change/oversight, and even then, that won't save you from direct attacks. Plus, even with legendary resistance plenty of higher level monsters can force multiple saves, and others can knock you out without a save. Monsters are just way more dangerous. Maybe legendary resistance is the only way this spell isn't trash, actually.