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Ahh... welcome to the world of programming.
You'll have to dissect both DBs and perform a mapping. I can pretty much guarantee that your Forums will be all but impossible to convert. The DB on the XOOPs side has changed A LOT since v1.
You might be better off just srchiving the old Forums and starting over. Next time, I'd recommend keeping things a bit more current than the every 4 year upgrade.
Pax vobiscum,
...mark
may the road rise to meet your feet!
http://treemagic.org