Today I did it - I've updated my MySQL installation on my production system to MySQL 5, to be more concrete, to MySQL 5.0.13-rc-nt-max.
That was not what I initially planned. I thought, I'd wait until MySQL becomes a Production Release, but everything works fine, I did a lot of things on my testing environments and I didn't find any bugs. So I asked to myself, why wait ;-)?
It was quite an easy move, even easier as from 4.0 to 4.1. From 4.0 to 4.1 I had to try hard to find the character set and collation settings that work best for me (as German speaking I need umlauts). But now from 4.1 to 5.0, everything was just smooth.
I imported my mysql privilege database from my 4.1 server to a test server with 5.0. I copied MySQL 5.0's mysql database and truncated the data from the db, tables_priv and user tables. Then I made SQL dumps from these tables from the mysql 4.1 database with extended inserts, but without the table structure. I only had to import this dump into my cleaned up mysql 5.0 privilege database.
Finally, I manually changed the privilege settings for the new privileges that exist in MySQL 5.0. But that was quite easy to do with simple SQL strings.
A 'flush privileges' finished my short piece of work.