Hi virtualgirl,
A GPO is self-contained w.r.t. the policy settings that need to be applied. ADMX templates are only used when creating or editing GPOs, or when viewing settings in the GPMC; they do not affect the actual applying of settings.
So (unless your customer's GP admins are continuously trying to edit the UEM/DEM GPO ), no downtime is required for this upgrade.
UEM/DEM ADMX templates are backwards compatible.