Remove it all. Start over with the bare studs and a new plumbing valve.

Thank you Menardi

I know but the valve can't be changed, otherwise the building management charges $1000 each time they do a water shutdown...

Only that is more expensive than the bathtub....

I want to keep the price as low as possible.

Could you please tell me, what would be the bare minimum in term of waterproofing given the gap between the current bathroom height and the new one.

Thamk you

What is the current waterproofing system used? You cannot lap over it going up if it is a membrane which is why it all needs top come out as the last thing you need is to be dumping water into the lower floors bathroom since this is a condo or apartment building.

I think the answer is maybe. You didn't mention whether this a tub/shower combo or just a tub. If it's a tub/shower, then yes, it must all come out and redone with new waterproofing. However, if it's just a tub, I suspect some pros might say you could just get away with removal of lower tiles, new waterproofing and replacing removed tiles.

"I understand what you mean but it is in a coop and you replace tub by a tub.

The person is going to use it as a shower.

so it is a good compromise."

From the OP's other post on the subject matter. This adds some context.

The only real way to prevent future water damage is to remove it all and start over.

Thank you very much McDonald

I'd need pictures and dimensioned drawings to offer an opinion, but I don't need any more information to know the usual "tear it out and start over" crowd here is doing their thing.

I tied a new shower seat and floor into the the walls 8 years ago and have yet to get a callback. That takes more skill and knowledge than starting from scratch.