We propose to study two phases of Na-NH3 intercalated FeSe which have distinct values of Tc, both close to 40 K, but different concentrations of NH3 and quite distinct crystal structures. We will perform the chemical transformation between these two phases in situ on the muon beam line. We will use transverse-field muon-spin rotation to probe these two phases in detail and extract information about the superconducting ground state. We will also demonstrate that the transformation, which is effected by absorption and desorption of ammonia, can be carried out reversibly.