Existing Xen Media Gallery customers can upgrade to XenForo Media Gallery following the usual add-on upgrade procedures. Download the latest version of XFMG from the your customer area and upload the new files over top of the old ones. Then, from your XenForo control panel, choose to update Xen Media Gallery and enter library/XenGallery/addon-XenGallery.xml as the local file. Once you submit the form, the upgrade process will begin. Once the rebuilds have completed, your upgrade to XenForo Media Gallery will be finished.
There were some permissions that were still in there from an old version that were removed (that didn't do anything) and we went with less granularity for "per page" options. There were about 10 different settings for the number of media items / albums / comments per page, we simplified these down to 3.