I love the latest E-Blah version and would love to upgrade my current forum to it.  I have two main issues however.  First, I've got a brown theme on 9.6 that matches perfectly with the rest of my website, is there a similar one for 10.3.5?  Another thing that worries me about upgrading is that I've got nearly three years worth of posts and threads on my forums, I'd hate to lose all of that in an upgrade error.  I've done upgrades in the past, but this seems like a more involved upgrade.  Any input would be great on these issues.  You can view my forums at http://www.capitaljeeps.com/cgi-bin/forum/Blah.pl

EDIT: Thinking about this, would it be an issue to install a fresh version of 10.3.5 elsewhere on my server and just import the threads and users over?  This way I can tinker out any bugs without messing with the current forum and then when read just change the link from the old forum to the new one.  Is it possible to import all of the users and threads flawlessly from 9.6 to 10.3.5?

Backup your forum before anything.  I do upgrades for $20, if you would like that.  I do not fix the layout for that though.  I'm sure someone could make you a theme with the brown, or you could use the theme creator pcmantinker has created:

http://www.eblah.com/forum/m-1172292826/

If you have any problems with the upgrade, usually they are fixable -- but always have a backup.

So would I have issues if I installed 10.3.5 while running 9.6 at the same time on my server even if they are in different folders?  After looking at the files and everything I would guess no, however I'm unsure what happens when you run the setup.pl script.  I just want to install 10.3.5 on my sever without messing up 9.6.  I don't want the new version to overwrite some of the old version by mistake.

As long as they are in separate directories, you should be fine.