It is not supported to upgrade the base OS with Lync or Skype for Business Server installed.
All servers within a pool must run the same OS.
Paired pools must run the same OS.
Attempting to do so will cause problems, up to and including catastrophic failure of the pool.
The recommendation is to build a new pool to replace the existing pool and move users to the new pool. The previous hardware or resources can be used once the old pool has been drained of all users and workloads; this is known as the 'swing' upgrade method. Effectively, it would be a similar process to migrating Lync or Skype for Business Server versions.