i dont agree with this method, i would go for email reset password to left those dummy accounts and the forum will be clean
But what about legitimate users who have a now invalid email? Why are you leaving them out?
Probably they will put a short message saying to change email and put a link to thread. Also, this might also be available. IMHO it is better not to give access to passwords.
=snip= A secondary (maybe slightly-hidden) method will be to use your current password. So you needn't worry about having an invalid email address now.
A valid email address might be required in the new software, though. I'm not sure about that yet.