Campbell Scientific continues to provide the RF400 to RF401 upgrade as a service. There is a minor upgrade (pn 18206) and a major upgrade (pn 18207). As the RF401 is now retired, however, it may be a better investment to forego the upgrade and purchase a new, higher output, 250 mW RF401A radio instead. No upgrade service is available to convert an RF400 or RF401 to an RF401A.
