Just came across a post that made me want to bump this thread up:
Messenger Plus on Windows Live Messenger.
Could a solution be to redirect the old site (homepage only) automaticly to the new site and then have a link on the new site to the tutorials/feature listings for Plus!3?
If not that then atleast the new site should get a mention/link on the "old" one as people seem to remember/go to the old one to look for a versions for Windows Live Messenger.