I know you have so many challenges with cats and food but as this isn't Honey I'll say it anyway ... I've never transitioned anything and the change has never been the problem. They either like it and eat it or don't and won't, and I've never had to force anyone to eat what they don't want to eat. Molly's thyroid control food would have been the only problem as it was rather critical, but thankfully she loved it.
I've always seen it as very important to know that I can give something else if our brand changes or isn't available suddenly, at least to have some choices. Mine tend to be on one staple brand and then some 'treat' foods that they get one meal of now and again. We were on Felix with a weekly Gourmet Gold ... until the Felix out of the blue made both cat and dog very ill. So we fell back on the GG, bought a brand new box of Felix, gave Molly one pouch and wallop, what happened in the litter tray while I was in the bath was extremely funny but I can't tell it as I can't spell a liquid wet farty noise and can't share the delicate aroma either, but let's just say it was a very short bath after that and Madam is back on GG until I either run out of money or figure out what I would like to feed her regularly. So problem, instant switch, problem solved.