Thats where the fun starts! Although I accept it may not be to everyone's taste.
If you have solved 9a and say the first letter of that is a 1 letterer, then you know that that letter maps to the second letter of the new J. If its a 2 letterer then you know the first letter of its mapping is the same as the second letter of new J. Look at everywhere where you have crossing answers to start working out what must be the same as what.
I've solved just under half the clues and by doing that I have one entry that must be one of 9 words. So hoping to get some firm mappings soon.