Hi. I have just swopped my dog over to NI in preparation for getting our poo pup on Saturday. We did it over a week giving one meal of old food in morning then the NI in the evening.
My dog has a quite serious dietary condition but transferring her over to NI went very well. She has been on it only a few weeks but already her poos are good and there's a definite improvement in her coat and energy levels.
There are plenty more experienced who can advise better but I'd do it sooner rather than later, you'll be glad you did.
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.