I've got some bad news for you, if you're using Keepass2 in the new second generation kepass2 database format and you want to migrate to KeepassX in linux then you're in for a bad day.
KeepassX has no way of importing the new generation database and Keepass2 does not support exporting it to anything usable under linux. It appears their support of the first generation database format is dependent on the client running windows. I crawled through every script and work-around you can find on the net and none of them worked reliably.
That's that, pretty much. There's no native way. However, our old friend wine to the rescue yet again. You can download the windows version of Keepass2 and run it with wine with nary a problem (you might require .net in the wine prefix), export your data to a first generation database and then import that with KeepassX. It'll work as expected. Just don't go blind trying to google for a native way to do it, I did.