Upgrade to ArubaOS 3.4.5.3-FIPS first, from there you can upgrade to ArubaOS 6.3.1.2-FIPS.
Are you running an Aruba 3200 controller? Make sure it's an Aruba 3200XM or have a memory upgrade installed before going to 6.3.
Also see attached release notes for the upgrade path (page 68).