A DIAMOND STAR PLT; LNDG AT JYO; INADVERTENTLY CHANGED HIS ASSIGNED XPONDER CODE TO 1200; THEREBY VIOLATING THE DC ADIZ.

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FLT WAS FROM KBDR TO KJYO. VFR FLT PLAN WAS FILED WITH BRIDGEPORT FSS. ADIZ FLT PLAN WAS FILED WITH LEESBURG FSS. DID NOT OPEN FLT PLAN BECAUSE FLT FOLLOWING WAS USED. AT THE POINT OF ENTRY INTO THE ADIZ; I WAS IN CONTACT WITH POTOMAC APCH. JYO WAS IN SIGHT; THE CTLR TOLD ME TO CHANGE TO ADVISORY FREQ; STAY ON THE XPONDER CODE UNITL ON THE GND AT LEESBURG. I FLY INTO AND OUT OF THE ADIZ FREQUENTLY AND UNDERSTAND THE PROC. AFTER I WAS ON THE GND; THE FBO CALLED ME ON MY CELL PHONE TO TELL ME TO CALL POTOMAC TRACON. I TALKED TO THE OPS MGR ON DUTY WHO INFORMED ME THAT I HAD CHANGED MY XPONDER CODE TO 1200 PRIOR TO LNDG AT JYO. I COULD NOT BELIEVE THAT I HAD MADE THAT MISTAKE AND DO NOT HAVE ANY RECOLLECTION OF CHANGING MY XPONDER CODE. VFR PLTS ARE TRAINED TO CHANGE TO THE VFR CODE UPON CANCELLATION OF FLT FOLLOWING. IT IS THE PROC THAT IS USED IN MOST AREAS THAT I FLY. LEESBURG IS THE SECOND BUSIEST GA ARPT IN VIRGINIA. THIS PARTICULAR DAY WAS GOOD FLYING WX AND THE ARPT ENVIRONMENT WAS VERY BUSY. IF I DID CHANGE TO THE VFR CODE; IT WAS BECAUSE OF CONCENTRATION ON TFC SEPARATION AROUND THE ARPT. ATC MAKES IT VERY CLR TO THE PLTS TO MAINTAIN CURRENT XPONDER CODE. EDUCATIONAL SEMINARS ARE BEING SPONSORED BY THE FAA AND POTOMAC TRACON. ATTENDANCE SHOULD BE MADE MANDATORY TO THOSE PLTS THAT VIOLATE THE PROC TO PREVENT RECURRENCE OF THE VIOLATION.