There were some old scripts to do a conversion.
Try this one script
and see if it works.
You can check also this article on XOOPS France
And there is another script to convert from XOOPS to phpBB
, so you would need to reverse it to do the opposite.
Regarding User issues, there seems to be a problem - I've installed CBB 3.08 yesterday and had some issues with it, but since I was doing it in order to update it to NewBB 4.3, I didn't pay attention.
The most important thing was that the update to 4.3 went very smoothly. There are still some light bugs, but overall, the transition to 4.3 was very good. You might try that....
From Ohio, USA