The IDP 3.1 flex image you obtained from the IOT Marketplace is an evaluation copy which cannot be upgraded to a newer version of RCPL.
However, I would suggest you to upgrade to PRO version to unlock this feature.
Intel Customer Support
Did the license for Flex change? Because I have been able to upgrade my flex from 13... all the way through till 19 and maintaining it as Flex.
Furthermore, I have gateways that have been upgraded to Pro. I have not been able to upgrade them to newer version because of the severe lockdown that is put into place as part of the Flex to Pro upgrade.