PLT OF C172 BELIEVES HE MAY HAVE INADVERTENTLY SWITCHED XPONDER CODE TO VFR WHEN TERMINATING ADIZ FLT PLAN WITH PCT TRACON; ALTHOUGH STILL WITHIN THE ADIZ.

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I WAS RETURNING TO MY HOME BASE OF GAITHERSBURG; MD ON AN ADIZ FLT PLAN. AS A NEW PLT WITH ONLY 1 YEAR OF EXPERIENCE; I HAVE LIMITED EXPERIENCE TALKING TO ATC. ONCE GAI WAS IN SIGHT; ATC TOLD ME THAT RADAR SERVICE WAS TERMINATED AND FREQUENCY CHANGE APPROVED. NORMALLY; I WOULD CHANGE THE RADIO AND XPONDER CODE AT THIS POINT. HOWEVER; I BELIEVE NEW ADIZ RULES REQUIRE MAINTAINING THE ASSIGNED ATC CODE UNTIL ON THE GND. WHILE SECURING THE AIRPLANE; I NOTICE THE XPONDER CODE HAD BEEN RESET TO 1200. WHILE I MAY HAVE CHANGED THIS AFTER I WAS ON THE GND; IT'S ALSO POSSIBLE THAT I CHANGED IT IN THE AIR WHILE CHANGING MY RADIO FREQUENCY. IF SO; THIS WAS AN INADVERTENT MISTAKE. WITHOUT THE ADIZ; I WOULD NOT HAVE BEEN TALKING TO ATC. I UNDERSTAND THE NEED FOR SECURITY AROUND WASHINGTON; DC; BUT HAVE ALSO HEARD MANY STORIES ABOUT THE CONFUSION THIS HAS CAUSED. I BELIEVE SUGGESTIONS OF ASSIGNING SPECIFIC XPONDER CODES FOR INBOUND; AND PATTERN WORK AT EACH ARPT WITHIN THE ADIZ WOULD HELP BOTH PLTS AND ATC PERSONNEL. EVEN THE RECOMMENDATION FOR A 1234 CODE THAT WAS ACCEPTED FOR PATTERN WORK AT TOWERED ARPTS WOULD BENEFIT NON-TOWERED ARPTS LIKE GAITHERSBURG. THEN PLTS WOULD KNOW THAT THIS CODE MUST ALWAYS BE USED WHILE ENTERING THE ARPT PATTERN.