Depending on how many AP-125s you have, a second option would be to try and get hold of some of the older APs that work both under 6.5.x and 6.4.x, such as the AP-105s or AP-205s. These are end of sale now but you might still be able to get hold of them.
You could then swap the AP-125s for the 105/205s as a temporary measure, then complete the controller upgrade and then swap out the temporary APs for the 315s. The temporary APs would still work so you can always use them elsewhere (depending on how many licences you have).
I now this sounds like a lot of fuss, but it will be much easier than having to split your controllers up and then having to recombine them. Staff time has its own costs, so if you are going to spend a considerable time testing and then reconfiguring your network, and then reversing the configuration again, you might find that the cost in staff time for this costs more than it would for putting in temporary APs.
One thing to bear in mind the AP-315s are much more advanced than the AP-125, for a start for full functionality they require 802.11at/POE+ power from your switch. If you switch does not supply POE+, you'll get reduced 2.4GHz performance amongst other things.
I hope that helps
Alexander.