One last point, about the practicalities of the situation. A retail Windows 7/8/8.1 licence upgraded to Windows 10 retains the transfer right. Even after July. Microsoft will most likely want to provide a practical solution for people to exercise that transfer right. We know that a fresh Windows 10 installation can be made with a Windows 7/8/8.1 retail key since the November update. Chances are, in order to exercise the transfer right, Microsoft will keep this mechanism even after July. So hold on to your older retail keys, as the practical means to prove your transfer right. You may still have to contact phone support when you are transferring.
I am well aware of that. However, since OP's keys come from OEM upgrades that would make them non-transferrable OEM licenses (and not retail, as he's claiming):
Haven't tried this, but if he installed W10 offline, backed it up, then used e.g. Technitium MAC Address Changer to provide a "hardware" change, then activated online, would this be enough to give a different digital entitlement for each pseudo MAC address?