Can you cast True Polymorph to turn a creature into an object, and then into a (new) creature? How would it work?

Question

I am DMing a level 20 "3-shot" set in the present day. I am trying to run the sessions as RAW as possible.

After our first session, a player whose character can cast True Polymorph wanted to polymorph a Commoner NPC into a stronger creature so they could enlist them to their cause.

The True Polymorph spell prevents turning one creature into another, more powerful, creature. However, the player asked if he could get around this by turning the target into an object first.

My instinct says that he could potentially do that, but by True Polymorphing a creature into an object – say, a rock – it becomes a rock. Therefore, when you cast it again it is functionally no different than casting True Polymorph on a rock to start with, meaning the creature has no memories of who or what they were as a human.

How would this work, by RAW?

Answer 1

True Polymorph has duration of concentration; up to one hour, or permanent. Thus, a commoner turned into stone is not just a stone. It's still a commoner with a spell on them.

The PHB rules for combining magical effects state:

The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect--such as the highest bonus--from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

So, when you cast a second True Polymorph on the commoner already true polymorphed into stone, the first True Polymorph ceases to function, and the second one works as if the first one were never there. In effect, you still have to work with the CR limit of the original creature, because you are polymorphing a creature into a creature.

If the second casting would end before the duration of the first one, the first one would resume. For example, if "True Polymorph into stone" is permanent, and you "True Polymorph into a goblin" your commoner, and drop concentration after 15 minutes, he will go back to being a stone. But until then, the first casting does not count, as if it weren't there.