The old website must operate under www.domain.com / remain live during development.
The old website runs forum software that has urls deeply embedded that make it believe it's www.domain.com - and its config produces links with absolute urls pointing to www.domain.com. We need to run a new server to take over the rest of www.domain.com, and proxy through the old website forum so existing urls are all maintained for SEO and bookmarks.
One small help on the old server the apache directive "ServerAlias old.domain.com" returns the correct page from the forum engine when that domain is used in requests, but with a 302 found header with Location: http:// www.domain.com/forum for each response. (This would be typical of any website with ServerName www.domain.com ServerAlias old.domain.com in apache.)
Mod_rewrite is not the way to go performance wise, as we have 500-1000 users 'live' at a time on the forum, so we're attempting to avoid it.
(This question is different from all other redirect questions found here - no other questions discuss keeping the old server using www.domain.com but not having DNS point at it. This situation requires the old server and forum software keep www.domain.com as its config.)