What I would suggest about the time issue is you close the old forum down as said before and put a message at that page saying the forum is down etc.
Now, with eukhost they will give you a temp address to go to the
website hosting while the dns is propogating which will be something like
http://84.84.84.84/~username. If you put this as a link on your old
website hosting (or a forwarder), anyone who hadn't had their dns updated would get the message and be able to click the temp link or getting taken to the new server automatically. Obviously you would have put the
website hosting up on the new server a.s.a.p. (it could be already set up before the change and you'd just have to import the database) which would minimise any downtime. Hopefully then it would be actually unavailable for around 1 hour max (it should be more like minutes really depending on the download and transfer of the database) even if people then had to maybe redirect themselves for the next 48 hours tops (but as I say, this could be done automatically with a redirect).
Hope that all makes sense.
