Ciao
You can safely proceed with the license downgrade from 7.x to 6.x and when you are ready redo the upgrade from 6.x to 7.x. The only thing you need to check is to always have an active "Support and Subscription contract".
You can find other posts in the forums where they talk about it:
https://communities.vmware.com/t5/vSphere-Upgrade-Install/Downgrade-vSphere-7-Enterprise-Plus-to-6-0...
and there is this KB that explains how to upgrade and downgrade and what constraints there are.
https://kb.vmware.com/s/article/81665
