I just read the release notes and i found something interesting
Im not an native english speaker and i don tknow if im undersanding correctly
Before upgrading to any version of ArubaOS 6.2.x.x, take note of these known upgrade caveats.
l When upgrading a controller from ArubaOS 3.4.4.3, or ArubaOS 5.0.3.3, or ArubaOS 6.0.1.3 to ArubaOS 6.2.1.6,
perform a two-step upgrade as mentioned below.
Customers running ArubaOS 3.x:
n ArubaOS 3.4.4.3 and above > ArubaOS 6.2.1.5 and above > ArubaOS 6.2.1.6 and above
Customers running ArubaOS 5.x:
n ArubaOS 5.0.3.3 and above > ArubaOS 6.2.1.5 and above > ArubaOS 6.2.1.6 and above
Customers running ArubaOS 6.0.x:
n ArubaOS 6.0.1.3 and above > ArubaOS 6.2.1.5 and above > ArubaOS 6.2.1.6 and above
As i understand you have to upgrade it to 6.2.15 first and then upgrade to 6.2.1.6
Cheers
Carlos