Yes, "greenboard" is comparatively 'old technology'...it didn't really work in the first place - keeping moisture-ridden areas "moisture-free" and nowadays it has been passed - like slow truck on a highway -by newer technolgies viz: Kerdi or RedGuard or many other membranes.
But since it is up, leave it there. Don't waste it. CBU might have been a better choice from the beginning but the end result will be the same -ESPECIALLY if you are considering Kerdi. Now is the perfect time to award yourself a waterproof shower vs a shower that'll need replacing some 5 years from now.
Schluter (Kerdi) is a product that works so well that drywall (not even greenboard) is recommeded by the manufacturer - Schluter. CBU is fine too, but drywall is their #1 choice! Since Kerdi is a waterproof
membrane, nothing goes through it and therefore you can put anything behind it - and drywall fits the bill. OK, greenboard if it's already up, it doesn't matter if it's green, blue or purple - all you need is structural support - not water resistance.
Greenboard is appropriate for the ceiling...so go ahead with that. BUT FIRST, find out whether whoever does the shower stall (a carpenter?) knows how to play with thinset and put up Kerdi. It's a piece of cake actually, so he should be willing to listen.
So, in summary, leave the greenboard - BUT ADD KERDI! Don't cheat yourself out of getting a 40 year shower warranty versus a 5 year "maybe".