[devicenull]

CreateInterfaceFn* findValidInterface(CreateInterfaceFn fac,const char * inter,unsigned int offset) {
	if (fac==NULL || inter==NULL || offset ==0) return NULL;
	CreateInterfaceFn *temp;

	temp = (CreateInterfaceFn*)fac(inter,NULL);
	if (!temp) {
		int sLenName=strlen(inter);
		char intName[256], newVer[256];
		memcpy(intName,inter,(sLenName>256?256:sLenName));
		unsigned int cVersion;
		cVersion = intName[sLenName-1]+(intName[sLenName-2]*10)+(intName[sLenName-3]*100);
		cVersion = atoi(&intName[sLenName-3]);
		intName[sLenName-3] = 0;
		for (cVersion=cVersion+offset;(cVersion>cVersion-offset && cVersion>0);cVersion--) {
			_snprintf(newVer,256,"%s%03d",intName,cVersion);
			temp = (CreateInterfaceFn *)fac(newVer,NULL);
			if (temp) break;
		}
	}
	return temp;
}

You use this as so.
To replace

m_ServerDll = (IServerGameDLL *)((ismm->serverFactory())(INTERFACEVERSION_SERVERGAMEDLL, NULL));

You would use

m_ServerDll = (IServerGameDLL *)findValidInterface(ismm->serverFactory(),INTERFACEVERSION_SERVERGAMEDLL,2);

In that way, as long as the interface available is either 2 above or below the one it was compiled against, you will get a valid interface pointer. It's still possible to get null's, if for example the interface was totally removed.

[BAILOPAN]

Perhaps it would be best to have some sort of table of version compatibilities so you knew what was available or not.

I know it sounds like work but it'd be neat to have like interfaces.xml which you could parse and the user could modify, to adapt to a game.

[devicenull]

It would be but, its a lot more code
This is the simplest thing to do at the moment, requiring no user action.