Upgrading from V3.0.x to V3.1 please read.
 

I have noticed that a good share of the problems and questions posted lately are a direct result of upgrading a V3.0.x site to a V3.1 site.

Many of the templates have changed and in turn variables have been changed, links added etc etc.

What this means is that by upgrading you will likely encounter a few problems IF you continue to use your old V3.0.xx templates.

With that in mind I highly recommend you take a few minutes and switch your site via admin to use the new V3.1 default template to make sure your site is functioning. Once convinced everything is working, then switch back to your old template. Then as you encounter errors make note of where they are and what template file is being used. Compare it to the new one and merge changes OR simply modify the new template file with the custom changes you made to the old file.

In many cases the template file wont be moded so you could just copy the new template file to your old template folder to get its functionality.

Upgrading from 3.0 to 3.1.x
 

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...

:)

Also just to add :)
::Language Pack
Normally all new language strings are added close to the top of the file and will be labeled with the version number for the change. That is in an effort to try and make it a little easier to find the differences. We only edit the english language file as well.

Keywords and Description TAGs
 

You need also convert your old 3.0.x header to the new 3.1.x form regarding the Keywords and Description tags:

File: /template/yourtemplate/layout.tpl.php

the old form:
the new form:
Otherwise, the keywords and site description will not be published for your website.

:)