I don't think there's any golden rules. See how you feel. When I got Dylan he had a bad tummy so I put him on chicken and rice to settle it while I waited for the ni and then went straight onto that. You might like to just settle him into his new home for a few days, then you could switch him. I haven't heard of any problems from people who've switched all in one go, but you could speak to ni and take their advice on the best way to do it.