Are you sure there were no other changes between the MMS APN settings and the normal APN settings? If there was another change (like a user/pass or access point name) then this trick may not work for you. For my carrier (AT&T US) all settings are identical except for the proxy. If this trick doesn't work for you, you may be one of the unfortunate few that are stuck needed two APNs, one for MMS and one for everything else.