Allah said,

إِلَّا مَن تَابَ وَآمَنَ وَعَمِلَ صَالِحًا ...

Except those who repent and believe and work righteousness.

This means, "Except those who recant from giving up the prayers and following the desires, for verily, Allah will accept their repentance, give them a good end and make them of those who inherit the Garden of Delight (Paradise).''

For this reason Allah says,

... فَأُوْلَئِكَ يَدْخُلُونَ الْجَنَّةَ وَلَا يُظْلَمُونَ شَيْئًا ﴿٦٠﴾

Such will enter Paradise and they will not be wronged in aught.

This is because repentance wipes away that which was before it.

In another Hadith, the Prophet said,

التَّائِبُ مِنَ الذَّنْبِ كَمَنْ لَاذَنْبَ لَه

The one who repents from sin is like he who has no sin.

Because of this, those who repent will not lose anything from the (good) deeds that they did. They will not be held accountable for what they did before their repentance, thus causing a decrease in their reward for deeds that they do after their repentance. That is because whatever they did before repenting is lost, forgotten and not taken to account. This is an honor from the Most Generous and a kindness from the Most Gentle. This is an exception that is made for these people, similar to Allah's statement in Surah Al-Furqan,

وَالَّذِينَ لاَ يَدْعُونَ مَعَ اللَّهِ إِلَـهَا ءَاخَرَ وَلاَ يَقْتُلُونَ النَّفْسَ الَّتِى حَرَّمَ اللَّهُ إِلاَّ بِالْحَقِّ

And those who invoke not any other god along with Allah, nor kill such person as Allah has forbidden, except for just cause...) until Allah's statement,

وَكَانَ اللَّهُ غَفُوراً رَّحِيماً

and Allah is Oft-Forgiving, Most Merciful. (25:68-70)