In the AirPort Admin Utility, select the main base station, press the configure button. Under the first heading (AirPort) the fourth box down, under AirPort Network and labelled "Name". This isn't the first "Name" box because that one should carry the name of the actual base station.
Fill in this second "Name" box and then update the base station, that should do it.
I think you'll lose your connection at this point and have to manually reconnect but that shouldn't be a problem.