The secondary cluster was on a slightly different release of AOS from previous updates that were not fully commited to the primary cluster.
The AOS and AHV went as expected and it is updated to a recommended version AOS 4.75.2 and AHV 20160601.44
The primary updated to AOS 18.104.22.168 fine and then when checked there was not an upgrade for the AHV to the same as the secondary.
Currently the AHV on the primary is 20160601.6 and attempts to manually download the AHV 20160601.44 to enable and update the cluster keeps failing with either a code error or incompatible version.
Best answer by briansuhr
Have you opened a ticket with support yet? I would recommend that course as they will guide you through any steps and ensure everything goes smoothly.