I know this is something quite old now, but as my first post I may as well be useful!
Dont upgrade from 7.0.x to 7.5.3 or anything less, you will cause alot of problems especially if using MailEnable Pro and it will take a long time to upgrade.
7.5.4 is due out this week, and they have resdesigned the upgrade process which should sort these issues out
Cheers,
Paul