If you are upgrading from Lync Server 2013 to Skype for Business Server 2015. The available disk space will be checked during the “Verifying upgrade readiness…” process. If there is less than 32GB of free disk space an error is displayed and the upgrade process will not begin. Free up disk space and then select Retry.
I guess that when you are running Lync 2010/2013 in a virtualized environment, you could extend the disk and perform the in-place upgrade.