The legal status does not change retroactively. If the flight was done according to the rules set out for private flight then you, as a pilot, are prohibited to

The legal status does not change retroactively. If the flight was done according to the rules set out for private flight then you, as a pilot, are prohibited to make any money from it (with some exceptions). Getting commercial license will not allow you to earn money on previous flights that were done as a private pilot.