About a month ago, I upgraded tried to upgrade my Gallery to version 2.2.1. I wasn't having any problems with the current version. There were no features in the new version that I wanted to use that weren't available in my current version. In fact, I didn't even know there was a new version — until I started getting daily emails from Fantastico telling me to upgrade.
For those that don't know, Fantastico is a tool which automates the installation of web applications (which when compared to a normal install pretty much equates to the difference between installing something in Windows vs. installing something through a Linux CLI). Having installed Gallery through Fantastico in a matter of seconds, I figured the upgrade would be just as easy. I logged into my control panel, went to Fantastico and clicked the upgrade button for Gallery. Because the Internet hates me, at some point the upgrade failed.
With almost 6,000 images posted, I got a little nervous. I went to my Gallery site and saw a page that said the site was in the middle of an upgrade and said I could login to complete the upgrade. At this point, I thought maybe there were certain steps that had to be done manually. I clicked my way through the upgrade and got a ton of errors. When I was done, I was back at the original screen telling me the site was in the middle of an upgrade. I tried to walk through the steps again...and again...and again... This is when I learned that 2.2 is not backward-compatible with 2.0 — not at all.
I spent a full week trying everything I could and eventually got to the point where only half of the site was broken. The albums showed up (with most thumbnails being broken), but if I tried to go to any of the admin pages, I got the screen telling me the site was in the middle of an upgrade. I eventually broke down and did a fresh install of Gallery, then manually moved over all my albums. This was extremely tricky to accomplish because of the major differences in how the two versions work.
I spent the past week moving data, manually setting all of the album thumbnails because I couldn't figure out any way to automate that data migration, installing plug-ins to regain the old functionality, etc.. Tonight I finished the first phase: getting all the albums moved over and displaying properly. Now all I have to do is re-create my custom theme, which I'm really looking forward to doing...