You should be okay just to attach cement board to the studs and tile. I see a couple of problems, though, and sooner or later this is all going to have to come apart and be overhauled. The first problem is that there's no way to have a continuous vapor barrier (if there's one at all). Secondly, if the rest of the tub surround is over greenboard, it should be replaced, anyway. It's way outdated.

We were not going to replace our cast iron tub in the kids bathroom which doubles as a guest bath when we have company. I have the mentality that if it's not broke, why fix it? We ripped everything else out and decided to replace the tub as well. Everything else in the bathroom would be new except the tub which was not new and shiny and more importantly, the drain was 25 years old. We decided to pull the tub. I would hate to have an issue in a few years and have to ruin all the new stuff we are putting in. My 12 year old and I had fun breaking up the tub with a sledge hammer until daddy came home and did the final whacks to split it in half. Once out, we decided to bite the expense of a larger tub. What was there was a 30 inch tub which was only about 9 inches deep to the overflow. The new tub is 32 inches wide, a nice shape lending to more room in it and allows 12 inches of water to the overflow. Those few inches were costly; doubling the expense of the tub, but man what a nice tub it is!

May be too late for this but I would also advise the photos of the walls as posted above and also have the "blocking" put in where you know that you want towel racks, etc. My GC doesn't seem to get too crazed by moving electrical and cutting out drywall. He says that's the way it has to be because visually, and even with the best planning, sconces don't always line up exactly the way you want, things look funky when you are in the field. It's good you are going to do a thorough walk through at this point though. Some changes can be made right now if you catch them. We are at the same place with insulation having just gone up and drywall being installed this week and next.

Tile is fashion. Tens of thousands of new choices are discontinued every year and 10,000 new ones are introduced every year. You’re not finding a Alfred Dunner plaid skirt from 1999’s collection, even at TJ Max. You might find it at Goodwill, if you had a network that connected all of the Goodwill’s in the country, and they cataloged their donations to a database. That no more exists than does a database of old discontinued tile. You can’t find replacement tiles for tiles discontinued in 2018, much less 2000. Not unless it was widely used by a tract builder, in a tract neighborhood, and every house comes with a box in the attic. It’s the worse kind of needle in the haystack. Because the haystack is 20 years old. Everything done in 2000 is now 20 years old. Time to assess what has stood that test of time, and what wasn’t a durable or attractive option, and needs to be redone with something else.