I was having issues with 6.5 and rather than reinstalling, I stupidly upgraded to 7.0, which doesn't see much of the server's hardware, including the drive it's installed on as a datastore, even though under 6.5 it did. My fault, I know. I have now downloaded 6.5 and reinstalled it over 7.0 and it sees the hardware and datastore... but for some reason, I can't even see the 7.0 license, which I'm sure doesn't work for 6.5 anyway,.. but if doing the upgrade changed the license key to 7... how do I downgrade it to 6.5? And if I don't need to downgrade it, but use my previous 6.5 license... how do I even get that back since when I click on anything that should point to my licenses the site just gives me an error and says that page isn't available.
You won't be able to revert to previous version of ESXi 6.5 due to partition changes to boot devices in ESXi 7.0 version.
Refer this KB article for more details: VMware Knowledge Base.
However, you can try to reinstall the ESXi 6.5 version and select option "Install ESXi, preserve VMFS datastore" during reinstallation process. However you might need to do all configurations manually.
If you have different ESXi 6.5 host along with VC you can make use of host profile and apply the configuration using host profile after setting up management network to connect the reinstalled ESXi host to vCenter Server.