Is it possible (without any disruptions) to change NETWORK_INTERFACE to a.b.c.*, to answer connection requests on both .100 and .190?
If BIND_ALL_INTERFACES is on (which is the default), the shared port daemon is already listening on all addresses. In this case, all that NETWORK_INTERFACE controls is which address the daemon advertises; if NETWORK_INTERFACE matches more than one interface, HTCondor will pick one (see the manual for our algorithm). Given that the IPs are all internal anyway, I'm not sure when it matters which IP the negotiator advertises.
If BIND_ALL_INTERFACES is off, HTCondor will only listen on a single interface; if NETWORK_INTERFACE specifies more than one IP, HTCondor chooses one for you (see the manual). HTCondor will also use that interface for outbound connections (instead of using whichever one the OS thinks is right).
In either case, one thing to consider is how your negotiator(s) authenticate to your schedd(s); on a purely internal network, it's reasonable for this to have been done by IP address. If you can update your schedds to allow the negotiator's new address, you're done; if not, you may need to disable BIND_ALL_INTERFACES and set NETWORK_INTERFACE to the old IP, and port-forward from the new address the old.