You attempt to transform a creature that you can see within range into a Beast. The target must succeed on a Wisdom saving throw or shape-shift into Beast form for the duration. That form can be any Beast you choose that has a Challenge Rating equal to or less than the target’s (or the target’s level if it doesn’t have a Challenge Rating). The target’s game statistics are replaced by the stat block of the chosen Beast, but the target retains its alignment, personality, creature type, Hit Points, and Hit Point Dice.
The target gains a number of Temporary Hit Points equal to the Hit Points of the Beast form. The spell ends early on the target if it has no Temporary Hit Points left.
The target is limited in the actions it can perform by the anatomy of its new form, and it can’t speak or cast spells.
The target’s gear melds into the new form. The creature can’t use or otherwise benefit from any of that equipment.
* - (a caterpillar cocoon)
So the nerf to this spell is that temporary hit points can't be healed, but if someone does something that gives the target more temp hit points than what they'd have in their beast form will that still let them keep their polymorph?
RAW, it doesn’t specify that the spell ends if it has no temporary hit points from the spell, just if it has no temporary hit points. So I take that as meaning no the spell doesn’t end if the creature replaces its temp hp.
It doesn't specify that you lose the temp HP when you lose concentration. So couldn't I just use this to polymorph into a t-rex, get a 130+ temp HP, drop concentration and keep the temp HP?
This change made it seem like you couldn't target allies anymore but they made it a specific rule now you can chose to fail a save if you want.
So hypothetically I could turn the Ancient White Dragon BBEG into a caterpillar, trap him in a jar for an hour, and then throw him into a volcano and watch him burn.
Depends on the GM