The clothing tag is much more functional in nature, whereas fashion is not necessarily functional. You could ask the question "What was the clothing of choice for 16th century foot soldiers?" and the question "What was the fashion aesthetic of choice for 16th century foot soldiers?" and you aren't asking the same same question. I don't really feel like one is a subset of the either.
Your question is analogous to saying that "buildings" and "architecture" are synonyms. I think the two should remain separate and would not support a retagging campaign.
Looking the two tags over, there is a bit of a difference. There are some questions tagged clothing that it would be a stretch to call "fashion", eg: the question asking if Celts really didn't wear any into battle, and many under fashion that aren't specifically clothing related, for example the question about flappers.
That being said, I see clothing as very nearly a subset of fashion. By that logic, I don't suppose I'd have a huge problem if someone went on a retagging campaign to make them all simply fashion.