You can try changing the steam.inf file, but your clients have to use the same protocol
L4D2 file example:
Spoiler
PHP Code:
// NetworkVersion is the version in the wire protcol between client<->server,
// its not used for matchmaking, PatchVersion is used for that to ensure
// we don't get cross connecting to different releases. Only bump this if
// you know an incompatible change has happened to the network protocol in
// game and old demos will not work.
NetworkVersion=2.1.0.0
PatchVersion=2.2.2.0
ProductName=left4dead2
appID=550