I don't think he meant "change to one of your alts", regardless of what account it is on. I think he meant to *hand it off to another player*.

I knew [ 1 ] was possible, because I tried it with an alt after we became [ONE]. I approached a dev, and was informed that spaces were supposed to be clipped from guild tags (it is a sort of overlooked bug that they are not), so he was unwilling to change our guild tag for me.