Your current EV version 9.0 SP3 & you can upgrade directly to EV 10.0 sp4 (latest version) as per upgrade path supplied by Gabe.
If your current server is 32 BIT then you need to migrate to 64 BIT hardware which also upgrade to 10.0.x version at the same time. The details and steps are mention in guide (Enterprise Vault Server Settings Migration wizard) http://www.symantec.com/docs/HOWTO42445
You also need to move to different domain (AD) domain which you achive by following the Guide mention in TN http://www.symantec.com/docs/HOWTO77280 (How To Move an Enterprise Vault Server to a New Active Directory Domain).
Above information would give details of each phase (upgrade, migrate-64 bit hardware, migrate to another AD domain). Now it's up to you on how to take these steps, as example.
1. First move EV to different domain with current version (9.0 SP3) on 64 BIT hardware.
2. Upgrade to latest version (10.0 sp4)
Upgrade instruction already given by other members.