When getting my new horse, and moving from hay to haylage...?

ok so i'm getting a new horse, the one i'm hopefully having has eaten hay his whole life, never touched haylage, but at my livery the only option is haylage unless i buy my own hay. I'd rather just keep him on haylage, as i have to pay for it no matter what. I've seen horses come onto the yard and get bad diarrhea, even colic, when they move onto haylage. Would it work if i bought a small bale of hay, and mixed it with the haylage for a while before moving him completely onto haylage? Or is there a better idea? thanks x