For anyone considering the plunge I did an online upgrade from ESXi 6.7 to ESXi 7.0 last night and although it's early days my Unraid VM is working fine. I just followed this old guide http://www.virtubytes.com/2017/07/10/online-upgrade-esxi-6-5-using-esxcli/ and substituted ESXi-7.0 in the grep line when looking for the right image profile.
The first time I ran the upgrade I got a warning about incompatible hardware, which was a Dell Perc H310 controller. Since I'm passing that through to the Unraid VM is can be safely ignored. Re-running the update with the '--no-hardware-warning' switch ignores the warning and allows the upgrade to proceed.
My only issue after the upgrade was that I couldn't attach the Unraid USB stick to the VM. I eventually remembered that I'd previously been using the older vmklinux USB drivers which have now been removed from ESXi 7. Once I'd re-enabled the native USB drivers with
esxcli system module set -m=vmkusb -e=TRUE
and rebooted I could see all my USB devices and was able to attach the Unraid USB stick to the VM and boot it.
I was lucky that all my hardware was supported, but do some checking first before upgrading
there's advice out there for checking for anything using vmklinux drivers https://blogs.vmware.com/vsphere/2019/04/what-is-the-impact-of-the-vmklinux-driver-stack-deprecation.html
You can run the upgrade in dryrun mode to get a list of any incompatible hardware
Assume that recovery console won't work if you want to downgrade again. There's a lot of articles out there that suggest that you can just launch the recovery console to downgrade to the previously installed version, but for whatever reason that option wasn't available to me.
HW List: Gigabyte X570 Aorus Elite motherboard with a Dell PERC H310 in IT mode passed through to Unraid.