Can I remove a tub surround, without removing the tub?
I have done almost this exact same retrofit. It was a lot of work, but you can leave the existing tub. I refinished mine with a Rustoleum epoxy product, but that's another story.

Basically, depending on the tub, I would take everything down to the studs, and start with a good masonry board, and any possible structural/plumbing changes you might need.'

The important thing is to make sure when you are all done you've accomplished two things:


A good, consistent seal from the tiled surface to the bath tub
See No.1


I've repaired and refinished bathrooms where this was done, and 9 times out of 10 there was water penetration around the bathtub that caused unmentionable horrors that were invisible, until it all failed horribly.

Good luck and take your time.

PS - a "zip saw" or handheld router can be your friend to cut that modular shower into smaller pieces.