1. Remove tub
2. If you have 1 1/2" drain, you should upgrade to 2"( vent stays the way it is )
3. Install new trap and drain head
4. Install shower faucet ( 42"-48" high) and shower head outlet ( 72"-82" high).
5. Install shower pan / hot mop / liner
6. Once all walls and floor is finished, install faucet trim and shower head.

That's all that is needed from plumber...

PS: Kohler makes cast iron shower pan that fits exactly in place of old tub. It is made for situations like yours. Even the drains line up. Cost: about $400.00. Looks very sharp... Available in Lowe's / Special order Department