Currently, I have JIRA 6.2.6 and need to upgrade to 7.13.2 (and to 8.5 later). Current Prod is a Windows server 2008 Physical server and current Test is a VM. I will have 2 new VMs to work with. I am considering starting with a P2V of our Prod system, and then using that VM for upgrade to 6.4. ... then 7.0.11, still on Windows Server 2008. We will then migrate the 7.0.11 to one of the new VMs with Windows Server 2012. And then, upgrade to 7.13.2.
Alternatively, we could perform an in-place upgrade of 6.2.6 to 6.4, and then do the P2V, or new install migration to the VM, and then follow with upgrade to 7.0.11, as above.
One concern is that we need to keep our Prod JIRA up during all the upgrade testing and can only have downtime for the final upgrade on the Prod VM.