There is a four year limit on reopening SSDI decisions, either favorable or unfavorable, if the decision was not still under appeal. So the decision that was made 8.5 years ago cannot be reopened under the rules of administrative finality.

Onset date can change the monthly benefit amount (generally higher but can be lower) because there are fewer months in the PIA computation period. It may or may not change the months of retroactivity depending upon the date of filing since there can only be 12 months of retroactive payment from date of filing. Also, you may not have sufficient medical evidence dated 10 years ago to even prove that you were disabled then.

Bottom line: you are too late.