What happens to what came before
It applies from the point the person gets it right.
Say this
إِذَا أَسْلَمَ الْعَبْدُ فَحَسُنَ إِسْلاَمُهُ يُكَفِّرُ اللَّهُ عَنْهُ كُلَّ سَيِّئَةٍ كَانَ زَلَفَهَا
Idhā aslama-l-ʿabdu fa-ḥasuna islāmuh, yukaffiru-Llāhu ʿanhu kulla sayyiʾatin kāna zalafahā
“When a servant accepts Islam and makes his Islam good, Allah wipes out from him every bad deed he had put forward before.”
Where this comes from
Then the reckoning starts from there, with the good counted ten to seven hundred times and the bad counted once, unless Allah passes over it.
The full narration
Narrated Abu Sa'id Al Khudri:Allah's Messenger (ﷺ) said, "If a person embraces Islam sincerely, then Allah shall forgive all his past sins, and after that starts the settlement of accounts, the reward of his good deeds will be ten times to seven hundred times for each good deed and one evil deed will be recorded as it is unless Allah forgives it
Source
Ṣaḥīḥ al-Bukhārī · 41
- Recorded in Ṣaḥīḥ al-Bukhārī, whose narrations are accepted as authentic