If someone decides to change his lifestyle and became a true Muslim then how can his good deeds outweigh bad deeds?
مَن جَاءَ بِالْحَسَنَةِ فَلَهُ عَشْرُ أَمْثَالِهَا ۖ وَمَن جَاءَ بِالسَّيِّئَةِ فَلَا يُجْزَىٰ إِلَّا مِثْلَهَا وَهُمْ لَا يُظْلَمُونَ
Whoever comes [on the Day of Judgement] with a good deed will have ten times the like thereof [to his credit], and whoever comes with an evil deed will not be recompensed except the like thereof; and they will not be wronged.
مَن جَاءَ بِالْحَسَنَةِ فَلَهُ خَيْرٌ مِّنْهَا ۖ وَمَن جَاءَ بِالسَّيِّئَةِ فَلَا يُجْزَى الَّذِينَ عَمِلُوا السَّيِّئَاتِ إِلَّا مَا كَانُوا يَعْمَلُونَ
Whoever comes [on the Day of Judgement] with a good deed will have better than it; and whoever comes with an evil deed - then those who did evil deeds will not be recompensed except [as much as] what they used to do.
ثُمَّ بَدَّلْنَا مَكَانَ السَّيِّئَةِ الْحَسَنَةَ حَتَّىٰ عَفَوا وَّقَالُوا قَدْ مَسَّ آبَاءَنَا الضَّرَّاءُ وَالسَّرَّاءُ فَأَخَذْنَاهُم بَغْتَةً وَهُمْ لَا يَشْعُرُونَ
Then We exchanged in place of the bad [condition], good, until they increased [and prospered] and said, "Our fathers [also] were touched with hardship and ease." So We seized them suddenly while they did not perceive.
وَلَا تَسْتَوِي الْحَسَنَةُ وَلَا السَّيِّئَةُ ۚ ادْفَعْ بِالَّتِي هِيَ أَحْسَنُ فَإِذَا الَّذِي بَيْنَكَ وَبَيْنَهُ عَدَاوَةٌ كَأَنَّهُ وَلِيٌّ حَمِيمٌ
And not equal are the good deed and the bad. Repel [evil] by that [deed] which is better; and thereupon the one whom between you and him is enmity [will become] as though he was a devoted friend.
I hope these ayats helps you understand how a good deed outweighs bad deed.