To let players with any version join the server this function can be hooked:
and client version in C2S_CONNECT should be changed to the server's version. That will work if the server and client versions have identical exchange protocol. If they are not identical, the behavior will be unpredictable.
One more thing - structure netpacket_s in the SDK is outdated, buffer field should be shifted by 4 integers.
If you simply want to update your server, there is an instruction: https://developer.valvesoftware.com/wiki/SteamCMD
At the moment it is not possible to downgrade source servers.