It's under 25 characters, so it's not impacted by the extension in tag length.
I've added [big-eyes-small-mouth] as a synonym pointing to besm.
I've played BESM for a while, some years ago, and my impression when I did was that the acronym was used almost exclusively, similar to how everyone would say GURPS, never "Generic Universal Role Playing System".
True, BESM is not as easy to say as a word as GURPS, but I still feel it should stay as an acronym, and have the full name point to it.
Whether or not we do a rename, those two should be syns. Either a new tag should be created and the old one made a syn or the proposal should be made a syn of the current tag. I don't really care which and I don't think it much matters, but if there's a good reason I'm missing I'm sure it will come up in discussion.