If the MOH device is one that uses a tape you should be able to just eject the tape and load a new one. The device will play back the new tape and record it into digital memory. But note that some devices, depending on vintage, will use "leaderless" tapes (tapes without the clear or white blank tape at the end). If that's the case you may get some dead air on your digital recording each time it repeats...if so, you can create your own leaderless tape by opening one up and cutting off the leader.
Or leap into the 21st century and buy one that plays WAV or MP3 native. You also can get a license on the 5000 to use file-based MOH without any external device, it just loads on the PBX.