My horse is moving..help me prepare plz?

I am moving my 6 yo anglo-arabian to my fiance's house in a few weeks from my friends house where he has been on pasture 24/7. My fiance's sister has her $10,000 barrel horse there that she is getting ready to sell and they wnat my horse to be stalled untill they get her horse sold(her horse is very aggressive and we dont want them fighting)...my horse never gets grian b/c he is on about 20 acres of pasture and is fat...when I move him to my fiance's house, he will be getting bermuda hay and safe choice evveryday and will be worked everyday..so how do I go about getting him ready for the change? I know I cant just switch him over to a new grain over night or anything. He has had bermuda before but not regularly so how should I switch his diet to prepare him? He will only be stalled untill her horse gets sold and then he will go out in the day and be stalled at night. Hope I explaned enough :) thanks!!!!!!