The mesh access points (mesh point and portal) have a mesh PSK in their flash, that will survive an upgrade or downgrade. The mesh psk is only introduced into flash when mesh portals and points are "provisioned", so if you don't reprovision the APs, they will always connect to each other.
If you move the mesh Portal from a 6.x controller to the 8.x controller, the portal will upgrade its code, but still have the mesh PSK in its flash, so the mesh point on 6.x will still connect to it, get an ip address, attempt to discover a controller and upgrade/downgrade its code if necessary. The PSK for the mesh connection is burned into the flash of the AP, so it will survive upgrades and downgrades and the mesh portal will be able to connect to the mesh point. On the 8.x controller, I would make sure that the mesh cluster profile in the ap-group has the same PSK as the 6.x controller, so that if you have to reprovision a mesh AP, it will be able to reconnect to the network.
You need to make sure that the mesh point (the far end) ia not statically pointed to the 6.x controller, but uses an infrastructure discovery method, so that if it upgrades/downgrades, you know what controller will attempt to connect to after the upgrade/downgrade.
I would get two regular indoor APs and make them mesh portal and point and practice the migration in the lab, so you would know what to expect.
In short:
If you point mesh APs to a new controller via lms-ip, the mesh portal should upgrade, then the mesh point should upgrade and both should join the new 8.x network, unless there is something statically pointing them to the old 6.x system. One thing you need to be aware of, is if the mesh connection between access points is very weak, it could take quite some time for the mesh point to connect back to the mesh portal, download its code and recover, so you should be patient...