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if it's just users you want, then that should be pretty easy(ish) to modify the phpbb original to x-phpbb converter.
as newbb is loosely based on old phpbb, i don't think it should be too difficult either to migrate topics and all too.
jack_buu has made some instructions over on bbpixel.com, altho i think he would have been better posting them here rather than there..
there's a newbb2 to x-ipb convertor. and i also made a x-phpbb to x-ipb 1.4 convertor the other day, altho not tested.. still waiting for feedback from it.. (13 downloads and not 1 word to say whether it works or not)
i could probably manage to modify the phpbb to x-phpbb convertor to migrate to newbb, but that's the problem.. i have no phpbb forum and it's 1 hell of a time consuming job to install it, configure it, populate the forum and then try to convert.. so if any1 has a test server with phpbb on already that they could test with then let me know, and i'll see what i can do with making 1.