I have also played with my old tag from my old clan for months on XI servers & NOBODY bothered me about it.
I agree with both sides on this. you should not HAVE to change but you should show pride in your clan.
This one thing that screwed me up technically, is when I changed my tags I went from 49th place to 1201 on my stats!
So ...f you want a reason to not have to change your tags this one point carries alot of wieght, unless someone can put me back in my rating spot of 49th [or closer to #1 ]because I put alot of time & effort & now it's flushed down the toilet.