Just for confirmation: Upgrading SDX from 12.1 build 56.22 directly to the latest 13.1, with 13.0 vpx instance. The aim is to update the vpxs to 13.1 after updating the SDX server. Upgrading from SDX 12.1 build 56.22 to the latest version 13.1 is supported and will update everything automatically, including the LOM firmware, correct?

Yes, LOM firmware is updated automatically with the SDX bundle update. I only did SDX 13.0 to 13.1 upgrade. I usually do 1 major update version upgrade at the same time.

I have seen one go direct from SDX12.1 to 14.1 and there were no reported issues.