Hmm, for a long time I always thought, memorising pieces would come to me quite naturally by just playing them so often that I would have them in my fingers. Even today I feel that when I am able to play a piece decently I will always notice that the movements are deeply ingrained - I think there is no other way in learning a piece then ingraining the movements into your subconscious.
Nevertheless, most of the times, this "finger memorisation" won't work very well - one wrong note and you will not know how to go on (as I had to learn through some bitter experiences...)
Therefore, with the help of this forum, I have tried to organize the memorisation process. Of course you are right when you say that you will have to do a lot of focused work on the pieces you already know, BUT, you can also use several shortcuts...
Now, as you perhaps know, there are several kinds of memory- visual, auditive, "intellectual", finger memory...
Now, I am actually quite good at visually remembering the score - at least to the expand that it gives me a clumsy idea of where I am going. Finger memory is always a helper, but you have to control it. Another thing I do is analysing the piece. That means chord progression, for instance. You just know: Ah, here I have my C Major cadence, now we have here the G7 in the bass, now comes the Aminor etc. Next thing would be motivs, themes, variations. Bernhard has mentioned somewhere that nearly all music is based on repition to 90 %. If it were not, it would be unbearable to listen to. That means you just have to memorise the main theme, and then you just have to know what the composer is doing with it (you have to memorise the second theme as well, of course, you know what I mean).
But the best advice I read so far is the thing about making up a story of memory marks throughout the piece (there is this example of Bernhard where he goes to the supermarket and all the most fanciest, craziest things happen, just to show how one could memorize the shopping list, I definetly have to search it...).
Ah cool found it already. It is reply no.22
https://pianoforum.net/smf/index.php/topic,4321.msg66112.html#msg66112(ignore the posts of guirlandes or whatever, that was a very strange story...)
That is pretty cool, because so you will built up a lot memory hooks - let's say you start the piece and you are thinking of it as your daily routine: the main theme is you waking up, getting out of the bed, and as the piece develops you can make up a complete story for every section. When you then play it from memory just by the thought of like: what comes after the breakfast? Teeth brushing! you will remember your phrase (of course you do not want your boring routine for your sweet Scarlatti piece: you want a completely new story, with every detail you can think of - be creative!

).
Or you will know that it is just your theme a fifth upwards. Or that we are now in G Major. Or your fingers will tell you. But the important thing is you will have 4 or even a lot more memory hooks instead of just one (or none

).
Anyway, these are just some advices for memorising a piece you once knew (in case you did not already know this stuff, but anyway).
Now, if you are referring to the famous Bernhard's "polishing pieces"-method, I have to tell you, that it is as it is. Relearning completely from scratch. Devilish

Well, I do not know if this was helpful at all, so I will post the perhaps most valuable link at the entire forum:
https://pianoforum.net/smf/index.php/topic,5767.0.html(the master-list)

Just to re-read some things...
