If the re-sentencing is specifically because of the newly discovered prior conviction, the only likely thing that will happen is that you will get a harsher sentence. If you entered a plea to this charge based on a plea agreement you may be able to withdraw you plea and fight the case.
You should hire a DUI lawyer to help you with this.
Georgia has a ten year lookback period for DUI, so your 2009 arrest may have qualified as the 2nd in ten years for sentencing purposes. This depends on the actual dates of the arrests. You should contact a local DUI specialist who has knowledge of the local judges and prosecutors. I would suggest Jessica Towne out of Lawrenceville (770-338-2338), Gus McDonald out of Cornelia (706-778-7178), or Tom Barton out of Stockbridge (770-961-7400).
You can't plead first offender to a DUI in Georgia. It sounds like you entered a plea under the First Offender Act in 1999 to a felony, and the sentence hasn't yet expired on that original case.
If you can't get in touch with the PD who handled your case, you will need to hire an attorney to represent you in court.