[CarlitoBrigante]

Create a full DB AND file backup (you cannot imagine how often we were called because a client had deleted by error all his/her attachments when moving/upgrading a forum), first of all. Then import the backup into a different database and work on it. Make 100% sure that the configuration in your test directory is pointing to the test database, not to the main one.

Test the upgrade, then repeat the process in the live forums. There are obviously a thousand different ways to do things depending on a lot of factors, including how much downtime you can afford and so on.

For our clients we try to do as much as possible in the test directory so that we can move all the files to the main directory and repeat the upgrade on the main database.

Difficulties usually come from huge and poorly maintaned databases, or big databases on old machines, where updating a column in a table like the post table can take ages.

Once you have done this part of the work and you have upgraded in the main location of your website, it is time to start updating your templates and mods, one by one. This can take a lot of time depending on amount of customization done to your forums. You can use comparison tools like Beyond Compare (my favorite) or one of the free ones, like Win Compare (good but somewhat less user friendly than BC). If you are using advanced mods that required a lot of file editing and that changed completely the way some parts of the forums were rendered (like some of the tabbed hacks around, commercial and free) things might be more difficult.