I have two vampires who recently aged into young adults, and their favourite food did not change to plasma. I edited their hair and clothes via testingcheats create a sim when they aged up, however I have done the exact same with their father and it has not affected his favourite food as plasma/o positive.

It's a minor detail but its bugging me, any fix or will they be stuck with human food in their favourites? Thanks!

Currently the only solution I can think of is to cure them of being vamps, then turn them again as both testing cheats cas and the change of taste lifetime reward lack the o positive option

If you edit a vampire in CAS it is likely that they lose their plasma favourite, because it doesn't show in CAS. But this should fix itself after you saved, quit, and then reload that save.
If not then you can fix this with MasterController. It has an option to change a Sim's favourites.

Restarting the game did the trick, cheers