Information on Upgrading.
Backing up your database is <u>ESSENTIAL</u>
A guide on Codex can be found here:
A guide can also be found here:
The guides may differ slightly but both are safe to use. Please take your time and follow them - and if you are unsure, post here first.
There have been a huge number of changes (500 +) in the core. Studying Trac will show you all the changes if you are keen on reading that. There is not and will not be a changelog here. It is highly recommended that you fully replace the core files.
New in 2.0
Have a read:
There have been significant changes to the user levels. If you have different users, you must satisfy yourself how this will affect your blog. If you run any sort of plugin that restricts what different users see or can do depending on their level you must be confident about the upgrade. Check everything - do not come back here complaining about user levels if you have not checked and made absolutely sure you know exactly what will happen. The place you need to check is http://codex.wordpress.org/Roles_and_Capabilities
This page has details: http://codex.wordpress.org/User:Matt/2.0_Plugin_Compatibility
Check FIRST. Please don't upgrade and then say the plugins do not work. There are many plugins that break. Check first and if you need to contact the author, ask nicely :)
Deactivate plugins before upgrading. If you do not you may get errors and the advice here will be to delete those plugins. Your choice.
If you experience any, post to this thread. Please give us lots of information such as:
The precise error
What you have done to try and mend the blog
If you really did follow the full upgrade instructions. We can tell you know... :)
Also, do not just say "I have the same problem" as someone else - you probably haven't - you should state your problem in full, along with what you have done.
And please remember - all the help you get here is provided by other members.
(from the wonderful Podz)