INEXPERIENCED PVT PLT SQUAWKS VFR PREMATURELY UPON EXIT FROM DC ADIZ.

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AFTER DEPARTING MARTIN STATE; I CONTACTED POTOMAC APCH TO REQUEST FLT FOLLOWING. IMMEDIATELY AFTER MY REQUEST THE CTLR ALERTED 'ACFT 12 O'CLOCK POS; 3 MI.' AFTER SCANNING I SPOTTED AN ACFT; RPTED 'ACFT IN SIGHT' AND TOOK EVASIVE ACTION. THE CTLR THEN HANDED ME OVER TO 119.0. AT THIS POINT I ALSO CHANGED MY XPONDER (FOR EXITING THE ADIZ) TO 1200/VFR. BEFORE I IDENTED MYSELF TO THE NEW CTLR; HE ADVISED 'ACFT 12 O'CLOCK POS; 1 MI; 2700 FT; I CANNOT READ YOUR ALT.' IT WAS THE ORIGINAL ACFT BUT THERE HAD ACTUALLY BEEN 2 ACFT IN THE VICINITY. MY ALT WAS 2750 FT SO I CLBED. THE ACFT WAS SIGHTED AND PASSED UNDER AND TO OUR R. THE CTLR THEN ASKED IF I HAD SWITCHED XPONDER CODES AND I SAID THAT I SQUAWKED 1200 BECAUSE I WAS CLR OF THE ADIZ. HE ASKED IF I WAS TOLD TO DO SO AND I SAID NO. HE THEN TOLD ME TO GO BACK; BUT CONTINUED TO ASK WHAT I HAD BEEN SQUAWKING AND WHEN I SAID XXXX HE TOLD ME NOT TO GO BACK TO THAT. FINALLY HE ASSIGNED A NEW SQUAWK. I BELIEVE I SHOULD NOT HAVE CHANGED XPONDER CODES WITHOUT ASKING. HOWEVER; AT THE TIME I WAS NOT SURE IF I ACTUALLY HAD BEEN ACTIVATED FOR FLT FOLLOWING. THE FIRST CTLR DID CONFIRM 'RADAR CONTACT;' BUT SINCE I DID NOT HAVE A DISCRETE XPONDER CODE I DID NOT BELIEVE I ACTUALLY HAD FLT FOLLOWING.