his is true but you can't change the version in the AMC without active SWA.
So a customer with active SWA who is entitled to upgrade to the latest release (e.g. 9.0) but is still on 8.0, after the SWAS is expired the customer is not entitled to upgrade to 9.0 any more?
Is there any document that describes this?
Document? This is just how it works... When you upgrade and change the AMC record, you must have active software assurance at that time.
Imagine you have a car with a 36,000 mile bumper to bumper warranty, and your radio (or whatever) fails at 30,000 miles, but you don't take it to the dealer to get it fixed until you have 50,000 miles on your car... Will they still fix it for free? Nope, you are buying a radio now. The same would apply for a non-safety issue firmware update to your car... If it is in warranty it's free, if the warranty is expired then you have to pay to have it done.