Which first: upgrade mysql version or convert storage engine? I inherited a production mysql server at 5.0. It uses MyISAM exclusively. It is not a high volume database but it does have some large datasets.

I am planning on:


upgrading to 5.5
converting existing databases to innodb


I will only do one at a time:

upgrade --> wait --> convert

or 

convert --> wait --> upgrade  

What is best to do first?


Upgrade to 5.5
Convert to InnoDB?
Of the choices, I would update, then convert.

InnoDB received some massive performance boosts in 5.5, so by upgrading first you get some features that 5.5 introduces, and then you can convert to InnoDB to get full innodb benefit. 

5.0 InnoDB performance was not great, so you might see some strange issues by converting to InnoDB prior to updating to 5.5

As always though, read through the MySQL 5.5 upgrade notes to make sure there's nothing new that would break your current implementation.

And, of course, test it out on a non-production database first.