I'm trying to figure out the best way to eliminate downtime when I move my forum to a new host.
I have never had to move a site before so I don't know what's going to happen? How long does it take for the DNS changes to update? I don't want people to type in the forum's url and not be able to get the site?
I should add that I am switching from phpbb to vBulletin and have the new software sitting on the .org name of the forum at the new host. Should I just tell people if they have a problem the day after I make the switch to go to the .org as a default?
If both sites are running at the time of the switch, all you'd need to do is change what your nameservers are pointing to and wait for the new DNS to propagate. However, this would mean that people on the old DNS will still be using the old server and posts could be lost. This is the easiest way to do things as long as you don't mind those few posts disappearing.
It's also possible to have the old site direct to the new site's IP address, but I'd imagine that's a little harder to setup (probably not too difficult though).
If you are switching hosts the problem will be some people will be hitting your old forum, while some will be hitting your new host - theoretically there will be no down time, just people reaching the wrong server - unless you take the page down. Something you could consider doing is putting up a redirection page on the old server at the time of the switch, so they are redirected to your .org page (obviously their DNS wouldn't have updated yet, people who's DNS is up to date would be loading the proper page from the new host).
What you can do to minimize this effect is to change the TTL (Time To Live) for the domain, essentially what this does is tells a DNS server to cache your DNS for a shorter period. However, you will need to wait however long it is currently set to before you can assume the new TTL has been picked up.