Hoping to benefit from the experience of others. This question is really multiple potential questions so I'm hoping someone can steer me to the best solution to keep me from all the trial and error.
So I have set up a WordPress v3.0.3 site currently with about 20 users and about 25 pages of content and I realize I needed a discussion forum for which I think the P2 Theme will work really well. I installed another copy of WordPress in the
/discuss/ subdirectory and then went about trying to share the user tables by setting
CUSTOM_USER_META_TABLE only to find that setting those constants are not sufficient because of issues with the
COOKIE_PATH, issues with capabilities that would require hacking core to fix, and it seems lack of support from the core team for multiple sites with the same user tables. So I fear heading down a rabbit hole of lost time trying to make this work.
So I thought maybe Multisite would be the solution which is unfortunately something that I've had very little experience with yet. However it seems that I can't just convert to multisite (according to @EAMann) but instead need to create a new install and then rebuild from there. Not knowing what that will break (i.e. will my users roles and capabilities transfer? Will the members plugin I'm using work? Are there any other concerns?) I'm also afraid to take that path unless I know it'll work well.
So in summary, what's the best way to get a site set up to have a main site and to use the P2 Theme for discussions in a subdirectory of the main site, to be able to use one user database and ideally to have a single login? (though single login is the least important consideration at the moment.)
Thanks in advance for the help.