As solar1951 says, upgrading should not be much of a problem if your /home folder is on another partition. If is isn't on another partition, there's a nice howto for moving your /home folder to a new partition on the wiki: http://www.linuxmint.com/wiki/index.php/Move_home_to_its_own_partition
The only thing I would add is that when/if you decide to try and move your /home folder, make sure that you have automatic network browsing turned off. I didn't think about that when I tried moving my /home folder, and it tried to move everything on my network (which is in the Network folder in /home). Needless to say, the partition got full before it could copy everything!
I'm actually running on Daryna Xfce right now, but if I remember correctly, you turn off automatic network browsing in mintDesktop, and then reboot. You can then turn it back on after your move.
About the Gimp, there seems to be no easy way to get a 2.4 version on a Feisty system. The .deb that solar1951 posted the link to is for Gutsy, so as he points out, you will run into dependency problems. I did a bit of googling, and there were many people asking how to get 2.4 on Feisty, but there were few answers. The best thing I could find was a tutorial on the Ubuntu community site: https://help.ubuntu.com/community/GIMP2.4-on-Feisty
It is rather long and complicated, and there is the risk of messing up other programs that depend on the files you would have to update for the Gimp.
Hope this helps out!