Would you explain more about the ideas of a convert being a new person. Are the sins no longer there? Is anything that happened to them now nullified such as them being able to be a witness for something they saw before the conversion?
The idea that a convert is a new person applies halachically to family ties. A sister and brother who convert are no more sister and brother and according to Torah law may marry, though the Rabbis forbade it. If two brothers convert and one testifies about the other the testimony is valid. Similarly, a Ger does not inherit his father's estate, even if the father converts. On the other hand, debts incurred before conversion are not erased. A Ger must pay the debts he incurred and may collect his previous debts. Similarly, a Ger does not gain automatic atonement for previous sins, though the conversion may be part of the repentance process. I know no reason that a Ger cannot give evidence on a matter he saw before the conversion.