IvorKovac

Both Romulans and Klingons underwent an appearance change, the former occurred between the TOS movies and TNG, whereas the latter occurred between TOS and the first of the TOS movies. A canon explanation was created for the Klingons in STE, but there is nothing for the Romulans. Of course I have no canon authority, but what I can do is derive an in-universe explanation by treating the fictitious universe as actual history.

This is my theory on how the Borg got started. Of course it's not cannon, but as there is no cannon explanation my theory has just as much authority as anyone else's. As I have stated before, Star Trek is a system of mythology, and as such it has it's own history and rules. Whenever you know history it's possible to make predictions, both about the future and past (in the real world as well as a fictitious world).