Whenever you upgrade you should read all the release notes for versions between your old version and the new one in our blog. Also you should see this sticky thread about Important Skin Changes, it has notes about needed skin changes that will fix your gallery.
See also Upgrading From Previous Versions.
However, also note that the demo site is running a newer build than 2.3.6.2 and the gallery has changed to use colorbox instead of greybox, so your gallery won't be quite the same until you upgrade to the next release of mojoportal coming out next week.
Hope it helps,
Joe