Trouble Importing "Too Old" WXR File

So basically, I keep running into issues trying to import an old WordPress.com blog that I exported to an XML file. Every time I try to import it, it gets all my authors, but says in a grey bar at the top:

This WXR file (version 1.2) may not be supported by this version of the importer. Please consider updating.

When I ignore this, assign the old authors to my new user accounts, that’s where I always run into trouble. It simply doesn’t work. I don’t think it has anything to do with the size of the file, as the file is only 3.1MB and GoDaddy says my maximum is 8MB.

What am I doing wrong here? Is there a way to “upgrade” my XML (/WXR) file so it can be handled by the version of WordPress Importer that I’m using?

Thanks so much, can give any more details if needed…