While the first 2 shots were relatively close together the problems of brand-matching seem simpler than they would be many months later.
The bigger issue would be if "Original Antigenic Sin" [0] could impact the immune systems response, but that would be part of the testing prior to releasing the booster I'm sure.
presumably if this is an issue at all - it'd be because most people are "locked into" the spike protein as the original antigen, as opposed to those naturally infected and therefore carrying a immune repetoire against however many 10+ other proteins in sars-cov-2.