Passwords in UBB.Classic where plain text.
When you import to threads, the password is MD5ed, but it's still the same password...
So if someone knows the password that it was on UBB.C they will be able to login to UBB.T no problem (as threads compares the MD5 hashes when a login is requested).
Quite simply, the password in UBB.Threads is MD5ed as:
md5(talons);