You're a deva that magically polymorphed itself because the change shape description says it magically polymorphs you. Change shape also doesn't say you must concentrate to maintain it, which means you don't

I don't disagree with anything in that thread you linked. Top comment says

The stat block I have for an ancient brass dragon does not list any shape change ability so what follows does not necessarily apply.

While the deva change shape description wording is central to the question you're asking here, so I don't disagree, it's simply not relevant to what we're discussing.

Next top comment is

Well you can only have one concentration at a time

Which is again not helping us here, because deva change shape does not require concentration.

Next top level comment after that says

Change shape does not require concentration.

Which agrees with what I'm saying here, then it goes on to say

As for abilities, you assume the game statistics of the thing you've turned into so no class abilities either.

Which is absolutely correct, if you're true polymorphing into an ancient brass dragon and using the ancient brass dragon's change shape ability, which is what that thread is about. I shapechange works very differently then true polymorph and deva change shape works differently that ancient brass dragon change shape.

Finally, the last top level comment says

By RAIWLTTBBTSOP (Rules As I Would Like Them To Be Because That Sounds OverPowered) yes.

Which is also a fair opinion to hold, one that I respect, much better to simply say you don't like the rule if that's all it is. I am definitely guilty of focusing too much on what a rule says, and not enough on what people wish it said.