Upgrading from 4.1 to 5.0 breaks things?

[Stever]

Hello,

I am in an environment that has a bunch of unrelated mysql databases
that exist on one server.  I would like to upgrade to mysql 5.0 to take
advantage of the newer features,  but I don't want to break existing
functionality. I checked out
http://dev.mysql.com/doc/refman/5.0/en/upgrading-from-4-1.html , but I
was wondering if anyone out there has done some real-world upgrading and
discovered that it breaks compatability with commonly known products
that use mysql, such as phpbb or wordpress or something like that.

Anyone have any real-world experiences of upgrading?

- Steve

PS I tried looking in the archives, but couldn't find much in this
specific topic.