Adding a couple of things that I encountered in my migration process:
::
Language Pack
DIFF your lang file over the english 3.1 one to check out all strings added. A very good, simple and free tool to do this is:
http://www.prestosoft.com/ps.asp?page=edp_examdiff
::
SEO Mod
If your are using the 3.0 SEO with your own custom made template, you will need to change it to reflect the new 3.1 SEO Mod.
To fix this you can search and replace (do it manually to make sure that the correct code is being changed):
<a href="viewlisting.php?view={$entry.id}">
with:
<a href="{$entry.link}">
::
Test all your websites' processes
It is obvious but... test placing an ad in every package, test the e-mail system (contact, registering, etc), test the navigation, and so on. I found a couple of glitches in my template (custom developed) that needed attention.
Hope this helps...
