I did the change kind of... medium speed
I did half and half RR mix/new mix for a couple of days, before changing completely. I know some people have had no issue changing mixes quickly, though.
If you're wanting more variety for Wispa, you might want to look in to JR farm (available on zooplus) and/or adding birdseed. (Watch out on the birdseed, though, most are really high in fat -- great for birds, not too great for hammies. Wilkos do a Finch one which has higher fibre and protein than fat, not something I've seen in most other birdseeds, and I know a lot of people use Trill budgie seed.) Both Burgess dwarf and JR farm I think have corn in, which Wispa can't have.