You tape the seams the same way you would if it were new drywall to new drywall. You either have to scrape the texture off and skim coat the whole ceiling or try and match the texture with the new ceiling. I would scrape and make the whole thing smooth if it were me.
Make sure you put cement board in shower area walls, or waterproof membrane over sheetrock. Don't just use regular sheetrock or greenboard.