Last week, the same customer requested an upgrade to Skype for Business Server 2015 and asked if it was possible to in-place upgrade their Lync Server 2013 pool to Skype for Business Server 2015. My first reaction was off course: No. You cannot. It is unsupported to have more than 1 legacy version installed before Skype4B.
Customer responded: I didn't ask if it is supported, I asked if it is possible.
So taking this request to the lab servers, I installed a new AD with a few user, a Lync Server 2010 SE enabled some users. Then a Lync Server 2013 SE and a Skype4B management server.
I started the Skype4B topology builder and downloaded the topology, and got to this result:
Nope, not possible... Clicked OK.
Tried anyway to upgrade my Lync Server 2013 pool:
Right click the pool and press Upgrade to Skype for Business Server 2015
Published the topology
This still gives me the creeps after 5 Skype4B upgrades
On the Lync Server 2013, I could mount the ISO file and start the installation (at least after adding another 20Gb of disk space to the VM).
In the end, all services are running and users can be moved to, and connect to, the upgraded server.
So, it is possible to migrate a migration scenario to Skype for Business Server 2015