I also changed my mirrors to the old style round GTS mirrors, not the Primavera ones and, yes, I needed adaptors. I also needed to use Loctite to glue the left one in, because when you adapt from right hand thread to left hand thread you can't fasten them properly.
Yes the rubber does no longer cover the threads, but I found a solution for that. I found something black plastic covers for acorn nuts in a hardware store, drilled a hole in them and sat them on top of the rubber. I don't have a picture, but this looks very good.