Allâh Subhanahu wa Ta'la will wipe out all the sins he had committed and write those good deeds in place of them which he will do in the future. He will efface the sins of the past and write the good deeds of the future. He will not leave it empty otherwise the angels will know that something is amiss. They will realize that something has been erased. Therefore Allâh Subhanahu wa Ta'la has protected the honour of His servants.
Allâh Subhanahu wa Ta'la will erase his past sins eg. a person who used to sing songs in a film has repented. He began performing salâh, sporting a beard and he performed hajj. Now whatever songs he had sung, Allâh Subhanahu wa Ta'la will write the talbiyah [Saying the words, "Labbayk Allâhumma Labbayk"] in place of it. As soon as he repents, Allâh Subhanahu wa Ta'la will wipe off his sins and write those goods which he will do in future. Is this not the grace of Allâh Subhanahu wa Ta'la?
