Is there a limitation on the support of older versions of SOLIDWORKS PDM Professional by Dassault and VARs?

My company supports several installations of SOLIDWORKS PDM Professional at various locations. Many of these divisions are satisfied with staying at older versions of SOLIDWORKS PDM Professional due to the disruption updates cause. I am trying to develop a model which would update the versions of SOLIDWORKS PDM Professional to the same across all of these locations.I am guessing there will be some push-back.

Is there a point where Dassault and VARs will not support older versions of SOLIDWORKS PDM Professional? The current SOLIDWORKS System Requirements Guide only lists SOLIDWORKS PDM Professional 2016, 2017 and 2018. Does SOLIDWORKS support SOLIDWORKS PDM Professional or EPDM on earlier version than 2016? How does the policy generally work with Dassualt and VARS for earlier versions?