Oh. Ew.
Yeah, I know what's going on. Not in an "I can fix it" kind of way, mind you, but I know how we got here.
Stupid question that I don't wanna test (because it has the possibility of a longer-term lockout) - Is there an "I forgot my password" option? Perhaps just having the site create a new password under the new system will work.
For those who care, my best guess is that the encryption algorithm used in the commercial software was licensed. That would mean that the Open Source version needs something else, meaning that no password can be recognized (and it can't be fixed). If that's the case, then resetting your password would create it so that it can be read properly.
In theory. Alternatively, doing that could put the password somewhere the software doesn't wanna look and make the account worse...