You can change the engine with phpMyAdmin. View the structure of the table you want to change and click the Operations tab. You will have the option to change the storage type.
Or you could enter this query into the sql window
Code: Select all
ALTER TABLE `phpbb_config` ENGINE = MYISAM
using your table prefix and the table name for the table you want to change.
I use InnoDB tables on a lot of my forums and never had a problem upgrading so I am not sure this is going to solve your issue. Since this is not a conversion issue, but an upgrade issue, ask in the support forum if you have any further questions.