Virtual machine hardware version is supported by vmware esxi 6.7
If there's any way you can get your current host to 5.5, that would be the least troublesome solution, since you could jump all the way to a supported version of 6 at that point, and further upgrades would be less drastic from there. No matter which way you slice it, you will definitely need at least a two-stage upgrade to get to a fully-supported version that won't need another full-stage upgrade within the next year or two, but expect that you'll need as much as a 3-stage upgrade to get current. If you cannot get up to ESXi 5.5 on your existing host, you'll need to stop at vCenter 6.0 to support your ESXI 5.1 configuration, and pull everything up to v6.0 from there, you will be able to look at upgrading to vCenter 6.5 or 6.7, which you could then get up to 7.0.
#Virtual machine hardware version is supported by vmware esxi 6.7 install#
From my initial analysis, you'll have to install no higher than vCenter 6.5 to support your old VMs if you can get them up to ESXi 5.5 on existing host for upgrading to newer versions. Your vCenter version will help, but vCenter 7 only supports ESXi 6.5 and up, so you would need to stop at a lower version of vCenter first to support your old VMs and upgrade them to newer versions before you could move up further. Much as they've suggested, I'll echo as well, that you're probably looking at trying to update to the newest version of 5.X you can get on your current hardware, and then move to ESXi 6.X on another piece of hardware that you can then update further to try and get into supported territory. As the others have said, 5.1 is so far out of date, that you're looking at a multi-stepped upgrade to try and get current.