How to update/upgrade Joomla! 1.5

I was compiling a quick-list with the Joomla! 1.6, 1.7 and 2.5 resources my developers and I used when we were converting ccInvoices and ccNewsletter to be Joomla! 1.6+ compatible, and thought the list might be useful to others. 

The resources are actually easy to find, you just need to know where to look (and who to follow on twitter). But for those in a hurry, here is our own "Joomla! 2.5 Resources" document. 

The section "Joomla! 2.5 extensions" applies mostly to extensions that are built from scratch for Joomla! 1.6+. the section "Joomla! 1.5+ extensions" applies to Joomla! 1.5 extensions that are being upgraded to also work in Joomla! 1.6 > 2.5. 

Check out our awesome Joomla! 2.5 extensions!

Generally important links

Joomla! Developer Network

Joomla! Development Strategy 

Joomla! Development Status 

Joomla! 2.5 Extensions

The best extension examples are probably in Joomla! itself. Just install the latest version and have a look at com_weblinks and other extensions.

Developing a Joomla! extension

Joomla! Developer Documentation
Joomla! Development - A beginners guide

What’s new in Joomla! for developers

Joomla! Framework classes

Potential backward compatibility issues in Joomla!

Language files

Database upgrade

Plugin events

Joomla! 1.5+ extensions

Converting an extension

Converting old extensions (check 1.5 to 1.6 section on top)

Tips on converting extensions

Making single installation packages for Joomla! 1.5 and 2.5

Upgrading a plugin

Version Independent Joomla Tips for a single package

Package extensions for Joomla 2.5 and 1.5 together

Languages and unified packages for Joomla! 1.5 and 2.5