On the surface this seems like a purely cosmetic change that I find difficult to justify given the knock on effects 
This was introduced in a major version which will always have API changes, otherwise we will have a lot of technical debt to deal with as the API evolves.
F_CALLBACK was being used for dsp state function pointers, which are not callbacks. see https://www.fmod.com/docs/2.03/api/plugin-api-dsp.html#fmod_dsp_state_functions
It is more appropriate to call it CALL to represent the calling convention (stdcall etc) rather than use the actual incorrect term callback.
A simple search/replace all will usually fix it in a short amount of time. hopefully that gives you enough background information!
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Thanks, that makes perfect sense of course.