From a techical point of view, the license key upgrade won't change anything in your environament.
However, once you upgrade the licese key, you have to agree that you will not use the old key anymore, so from that point of view you'd likely violate the license agreement. I'm not sure, if this will also have an impact in case you need support for the "old" version.
Please remember that you can run vSphere 7 in evaluation mode for 60 days, so that there's usually plenty of time to evaluate the new version, before you need to assign a license key.
André
