WFBS Advanced 9.5 only supports fresh install of Messaging Security Agent (MSA) and does not support direct upgrade.
Messaging Security Agent version 11.6 in WFBS 9.5 is mainly developed for Microsoft Exchange 2016. Hence, an old MSA may not need to be upgraded to 11.6.
The following scenarios show that you will still be able to install or manage MSA in WFBS Advanced 9.5:
Scenario 1: Doing a fresh install of MSA 11.6
When installing WFBS 9.5 Server, you can do a fresh install of MSA on an Exchange Server that does not have MSA.
Scenario 2: Retaining the current MSA version even after upgrading to 9.5
Those who have a previous version of WFBS (7.x, 8.x, 9.x) can upgrade the Security Server directly to 9.5. However, MSA will not be upgraded since we do not support MSA upgrade and it will stay in its current version.
Though MSA will not be upgraded, you should still be able to manage the previous MSA version from the WFBS 9.5 Management Console.
Scenario 3: Uninstall the older MSA and do a fresh install of MSA 11.6
After upgrading to WFBS 9.5, you may uninstall your previous MSA version and do a fresh install of MSA 11.6.