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

While the new temp hp rules say they last until depleted or until you finish a long rest, polymorph's temp hp are still an effect of a concentration spell. The temp hp wouldn't remain after the spell ended by lasting its full duration, so why would any effect last after you intentionally drop concentration?

Does this mean you can no longer permanently kill The Tarrasque by transforming it into a lizard and power word deathing it?

That is what I'm starting to realize lol. I'm going to put it in my game that if you drop concentration you don't keep the hit points from the beast form because. You are no longer a beast. I had a druid player a year ago where we simplified the mechanics of his wildshape to the temp HP of the creatures and every time he reverted out of his shape the HP would go with it. It seems like a big oversight that no one at WOTC caught this and created the adendum of "or concentration is broken from the caster"

Remember temp HP cannot stack. You get one or the other. 

Cast Armor of Agathys and Polymorph for some insanity. Neither cancels the other out, and the source of the temp HP is irrelevant. Legend tells of T-rex that survived the Ice Age and learned to harness it's power. This is that story.

Quote from Thecraftking >>

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.

 I mean if you manage to burn through all its legendary resistances, get it to fail its saving through, and the white dragon for some reason made its lair within 1 hour's travel of a volcano, sure why not?

Also RAW I don't think lava constrains movement at all, so realistically you will probably get 1 turn of damage and then the dragon flies out of the lava.

The temp hp wouldn't remain after the spell ended by lasting its full duration, so why would any effect last after you intentionally drop concentration?

That is your call / opinion,  the spell does not state that the temporary HP are removed when the spell ends.

RAW PHB 2024 for temp HP- Temporary Hit Points last until they’re depleted or you finish a Long Rest.

Probably not intended, but for now a great way to get 157 temp HP from Giant Gorilla or 189 temp HP from Sperm Whale.

I would say it doesn’t have to, since the temp hit points is an effect of the spell. Concentration states: Some spells and other effects require Concentration to remain active, as specified in their descriptions. If the effect’s creator loses Concentration, the effect ends.

No temp Hit points.

That’s not true. Rules for concentration clearly states that a spell effect ends after losing concentration:

‘Some spells and other effects require Concentration to remain active, as specified in their descriptions. If the effect’s creator loses Concentration, the effect ends.’

My main question around this.

Concentration can be dropped willingly by the castor.

But how does it affect this spell?

Is the castor in full concentration of this for the hour? Or does the castor get to concentrate on something else after its been successfully casted?

If the castor drops the concentration will the spell be nulled or be for the duration?

Quote from Starkyii >>

My main question around this.

Concentration can be dropped willingly by the castor.

But how does it affect this spell?

Is the castor in full concentration of this for the hour? Or does the castor get to concentrate on something else after its been successfully casted?

If the castor drops the concentration will the spell be nulled or be for the duration?

Ending concentration on a spell ends all the effects of that spell unless the spell itself says otherwise.

If you end concentration on this spell, the target goes back to being whatever they were before.

Quote from wagnarokkr >>

Quote from Starkyii >>

My main question around this.

Concentration can be dropped willingly by the castor.

But how does it affect this spell?

Is the castor in full concentration of this for the hour? Or does the castor get to concentrate on something else after its been successfully casted?

If the castor drops the concentration will the spell be nulled or be for the duration?

Ending concentration on a spell ends all the effects of that spell unless the spell itself says otherwise.

If you end concentration on this spell, the target goes back to being whatever they were before.

 But-

Temporary Hit Points

Some spells and other effects confer Temporary Hit Points, which are a buffer against losing actual Hit Points, as explained below.

Lose Temporary Hit Points First

If you have Temporary Hit Points and take damage, those points are lost first, and any leftover damage carries over to your Hit Points. For example, if you have 5 Temporary Hit Points and take 7 damage, you lose those points and then lose 2 Hit Points.

Duration

Temporary Hit Points last until they’re depleted or you finish a Long Rest (see the rules glossary).

=================

As I said above, probably not intended, but RAW the temp HP would last until depleted or a long rest.

This is most definitely a discussion to be had at your table with your DM, or if you are a DM this is a way for you to be ready when a player presents this to you.

I have a question.

So can you polymorph a willing participant into say a T-Rex?

Quote from Artorius >>

I have a question.

So can you polymorph a willing participant into say a T-Rex?

 Yes
Yes Indeed