Just checking.. Have been running XSI free, but bought the pro version for the new server that I am installing. The new server is running ESXi 6.7 and the old is running 5.5. The idea was to keep the old server as backup and run all production VMs on the new and then copy them nightly to the old, to serve as hot backups.
If possible, I will upgrade the old server to 6.7 also, but if this is not possible, can I run different versions of ESXi, provided i keep the virtual machine versions at a level, supported by both?
Yes, you can, just as long you keep the least common denominator, namely, as you stated: the hardware version the lower ESXi can run.
Nonetheless you will hit dificult issues to solve when trying to move data over an SSH tunnel that connects from some OpenSSH version to a much lower one, as many of the ciphers have been deprecated and newer OpenSSL versions include security features that some older ones won't have.
Thus, why complicating your life and try to do such cumbersome thing, which in the end you might very well not solve, when you can upgrade the old ESXi box.
You are absolutely right and the intention is to upgrade the old server as soon as I have all VMs moved to the new. I was just checking in case the HW will not allow it, but In that case I will try the most recent that runs.