If a PC uses the Magic Jar spell to possess another PC, what happens to their HP?

Question

My Necromancer is a level 13 player character, and he will use the magic jar spell on fellow 13th-level PCs. The issue is that HP for PCs is calculated and based on a class feature (monsters without classes are different than PCs).

The magic jar spell description says you retain the benefit of your own class features. If you benefit from your own class features, it seems like your hp should count. (Or am I wrong?)

There are 3 particular bodies in this question:

1. My original body - (is not discussed in this question)
2. Fellow PC's possessed body
3. Fellow PC's original body

What happens to body 3's HP when it turns into body 2?

What happens to the HP of body 2 when it turns into body 3?

Answer 1

You use the statistics, including hit points, of the creature you possess.

The description of the Magic Jar spell states, in part (emphasis mine):

Once you possess a creature's body, you control it. Your game statistics are replaced by the statistics of the creature, though you retain your alignment and your Intelligence, Wisdom, and Charisma scores. You retain the benefit of your own class features. If the target has any class levels, you can't use any of its class features.

Hit points are creature statistics and thus you use the hit points of the creature you possess. Current hit points, unlike hit dice, are not a class feature.

Your old hit points aren't necessarily gone; they are just not being used for anything during the possession. When you end the possession, you will again be using your hit points from before the possession – and the creature that was possessed uses its own hit points, including any damage done to them during the possession.

Note that since you keep your own class features, if you have any that alter hit points they are recalculated from the base hit points of the creature you possess.

Answer 2

I've always understood magic jar as a last ditch survival tool for mages, not a utility knife to borrow bodies. Most commonly parties will encounter a magic jar without a body left to return to. Or, as a last action, a mage will cast magic jar to escape and their body will bleed out and die leaving them trapped in the jar until they can forcibly possess someone. It is a poor-man's phylactery and much more fragile.

There are several vulnerabilities inherent in magic jars, such as the jar itself, which could be destroyed thus ending the spell, or dispelling magic on the jar also ending the spell. If either of those two things happen, hopefully both of you are within range of the jar or someone's going to die. Anti-Magic Shell. globe of invulnerability. A simple command spell telling you to dispel your magic jar. I dunno', there's probably more reasons to never do this than to play around with everyone's life like that.

In any event, the spell pretty clearly states you get the physical attributes of the host and maintain your mental attributes, Intelligence, Wisdom, and Charisma. By class features, it means to say you can cast spells, shoot a bow, sail a ship, etc...

If you want to homebrew a body-swapping magic, just do that. Magic Jar probably isn't what you're looking for.