You should have no problem changing, first seating is the one that fills up so generally it's easy to get second. Ask your TA to handle it or just call
DCL and request it. Shortly before the cruise you can see on the website somewhere what your seating is and if it got changed. We have second seating because first was already full- my DH really prefers first- but our other cabin doesn't care, so I think I'll just leave it alone. We had our TA put us on a waitlist for first seating on 3 cruises and got it twice. The third cruise we booked only about 90 days out so I'm sure we were pretty far down the waitlist by then.
The Exotic Rasul is a spa treatment that you do yourselves. It's for a couple. You go in this little room, where you are given a bowl of mud, various scrubs, etc and some instructions on which to use first, etc. It can be whatever you want it to be! Your attendant will give you instructions, and leave you alone for 50 minutes.

There is a steamroom on the right with private seats. You can add water to cool your feet or to create more steam. There is a shower on the left side of the main room as well. We have always enjoyed it and the lime scrub stuff is the BEST! Just be sure to bring a watch and put it in the pocket of the robe you wear to go in... I hated not knowing when the attendant was coming back. She'll knock when you have 5-10 minutes or so left, but I always worried I wouldn't hear her because by then I was in the shower. It's only $85 for the both of you so it's a bargain. We have always been able to use the rainforest room immediately afterward as well, which was nice. Once we got champagne afterward, maybe twice- but I don't think this last time we did. I can't remember!