Will a 64-bit SEP 12.1.4 GUP service 12.1.5 clients?

Simple question, my SEPm are all running 12.1 RU5 and a few test clients but my GUPs have not been upgraded from 12.1 RU4 yet.  So are my 12.1.5 clients still getting defs from the GUP or are they bypassing and getting defs from SEPm?  I haven't done any debug level on GUPs yet to check. Just need to know how urgent getting the GUPs upgraded is.

Thanks,