The reason that Change Instrument is non-functional in this project is that at some point in the past, you’ve created an extra staff for that instrument, which unfortunately rules it out of being able to have its instrument changed. Equally unfortunately there’s no good way for you to tell this yourself (I have to use my magic powers to divine this information).
What I suggest you do is create a new player with an Organ attached, copy and paste the music from the existing Organ player to the new player’s Organ, then delete the old player and its instrument. Now you should be able to change its instrument with impunity.