I am running an ASA 5520 on v 8.4 (2). Would I be able to migrate (upgrade) directly to Release 9.0.3.ED?
To my understanding it should not be a problem
The Release Notes for 9.0 Software says
Upgrade Path and Migrations
•If you are upgrading from a pre-8.3 release:
–See the Cisco ASA 5500 Migration Guide to Version 8.3 and Later for important information about migrating your configuration.
–You cannot upgrade directly to 9.0 or later. You must first upgrade to Version 8.3 or 8.4 for a successful migration.
As you can see, it first states that you check the other document if you running older than 8.3 sofware. It then states that you can't directly jump to 9.0 or later. It then follows to state that you first have to be in 8.3 or 8.4, which you are.
I constantly switch my own ASA between 8.4 and 9.0 and 9.1 software levels when testing configurations.