The thinner lighter weight drywall is used for ceilings. I imagine you could use the thinner stuff but I've never seen it used for walls. Probably not code, but don't know that. There are probably all sorts of logistical reasons for using the regular width typically used for walls.

I thought you were supposed to use 5/8 width on ceilings to prevent sagging?

Hmm..your right. I guess I learned something today about sagging on the drywall. What I thought I knew was wrong.

Found this: https://www.thesawguy.com/drywall-sizes/

"I thought you were supposed to use 5/8 width on ceilings to prevent sagging?"

There are millions of homes with 1/2" and absolutely no issues. I would definitely use the MR or Moisture Resistant.

Sagging has a lot to do with ceiling joist spacing. With the advent of trusses for roof framing, the 16” on center stick framed joists went to 24” on center. And you can’t really modify the bottom chords of a truss to add additional bracing without an OK from the engineer. A 24” span is a lot to ask of 1/2”. Especially in a humid climate with the typical poor air sealing into the attic that happens from builders. I prefer 5/8” X type for ceilings for greater rigidity.

My ceiling is in good shape. It's the walls that need replacing.

Is it possible to remove and replace the entire walls without disrupting the ceiling?

Possibly. From here with no other info it is impossible to be certain.

Enamel paint + good exhaust fan + 1/2" drywall.

Use 1/2" greenboard on your bathroom walls (not wet areas).