My experience shows that whenever I leave an organ piece to rest for a while (at least a few months), my knowledge of this composition decreases. I'm not saying that I would forget this piece entirely, though. It's not like I would need to learn it from scratch.
It's just I feel that I couldn't play this piece in public right away. If I did, I would probably make quite a few mistakes and feel unsecure. The pleasure of making music will not be there. I would have to really struggle to play the correct notes and rhythms with my hands and feet.
So whenever I start practicing this piece again after having left it to rest for some months, my first repetition is quite shaky. It's best to do it really slowly at first. This way my muscle memory will begin to come back.
The second repetition of this piece will still be very shaky but now in some places I would feel my skills coming back. The improvement wouldn't be easily noticeable for the listener, though.
With my third repetition, I would feel a lot better. Although I would still play at a relatively slow tempo, there would be much fewer mistakes. However, this is not enough for public performance.
Do this experiment for yourself. It will be interesting to see how long it will take for you to repeat your piece.
The subsequent few repetitions will make your playing even more secure. At about 10th run of the piece, you will start to feel your skills renewed.
Note that all of this valid only if you really knew this piece in the past. You did all the hard work of perfecting it. Perhaps you even memorized it. Only then my above explanations will work for you.
If you learned this piece a very long time ago, like decades ago, then obviously 10 repetitions will not be enough to remember it. We are talking about the period of several months up to a year at the most.
Apply my tips in your organ practice today. If you want to refresh the memory of the piece you once knew well, start out slowly, build up your technique by playing scales, arpeggios and other exercises regularly. Remember, you don't have to be ready to perform it in public within a day. Give yourself plenty of time in advance.
How do you go about refreshing your memory of the piece you mastered a long time ago? Share your thoughts in the comments.