2012R2 ROK upgrade VM?

Hello,

we have a old Proliant Server with a 2008R2 ROK license,

on which a Hyper-V instance (Named TSE) is running with the same 2008R2 ROK license.

The new Server is a Proliant G9 Server, with a 2012R2 ROK license.

We did take over the Hyper-V TSE instance to the new server.

This worked fine.

Now we wish to upgrade the Hyper-V TSE instance from 2008R2 to 2012R2

with the purchased ROK license, which is installed on the hardware.

From the licensing point of view this is allowed,

but when we try to upgrade the server, the upgrade process tells us

that upgrading to this build is disabled....

What are my options to solve this upgrade problem?

Could I upgrade with a non-rok image and then set the license key from the ROK license?