Replace faucet handles in existing bathtub
You will surely be able to remove the knobs without removing the faucet  valve body and  without removing the tub.

First examine the tops of the knobs and see if there are caps which pry off to expose bolt heads underneath. Remove these bolts (screws) and then wiggle the knobs off. You should be able to find other knobs more to your liking that fit the same shafts.