assuming you don't want the song to play for the second load then my first answer was the correct one - panel filters and data protection will have no effect.

Save registration (composer/panel memory type files) with a blank sequencer. Then when you load the next file the sequencer is emptied and there is no need to remember to switch off the play button, and no interference will occur to the new setup.

To achieve this, initialise the keyboard, load the registration file, and immediately save it with sequencer included with whatever other files (composer, panel memory, current panel etc) it originally contained.

To record a melody with a nx midi just load the file, go to sequencer menu, record & edit, realtime record, switch an empty track to red, go to mixer in screen and choose a voice etc for that track, press start, play song, then stop. Save with the original melody track muted, then you can switch yours off and have the original available if you want it in future. Or if all tracks are used, record over the melody track, or see which tracks can be advantageously merged to create an empty track.