In Keses HaSofer Siman 16 Sif 4 he brings the din of oisios gedoles and katanos that we are nohag to write in a Sefer Torah. He says that this is only "l'mitzvah" and if one doesn't make the proper letters "big" or "small" relative to the others that it isn't pasul. This is seemingly all in a situation where one must read from this Sefer Torah. What about after that situation passes and one now knows that the letters weren't written according to the minhag or mesorah, must one go back now and fix them? Is there any source for this?