MySQL version upgrade

[Jos Chrispijn]

I currently have running:

mysql-client-4.0.27 & mysql-server-4.0.27, who I would like to upgrade 
to mysql-client-5.0.75 & mysql-server-5.0.75

I can imagine that there is a database issue here as the 4.0 records 
might differ from the 5.0 ones.
Is there a way of upgrading smoothly? I did upgrade my databases in advance.

-- best Wishes for 2009 and thank you for your support in the last year.