C172 PLT CONDUCTS PUBLISHED APPROACH AND LATER DISCOVERS MDA HAD BEEN REVISED.

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I FILED AN IFR FLT PLAN AND RECEIVED A STANDARD BRIEFING FROM HAWTHORNE FSS. I SPECIFICALLY ASKED ABOUT NOTAMS AT MY DEST (CMA); ALTERNATE (DXR) AND WJF (VFR ARPT). MY FSS BRIEFER RPTED ONLY A TETHERED BALLOON AT CMA (052 DEG RADIAL -- 4 MI). APCHING CMA; I OBTAINED ASOS (NO ATIS AS TWR WAS LATE OPENING THIS DATE). NO MENTION ON ASOS OF REVISED MINIMUMS FOR THE VOR RWY 26 APCH. MUGU APCH CLRED ME FOR THE CMA VOR RWY 26 APCH AND ASKED ME TO CALL THEM ON THE GND ON THEIR FREQ (124.7). PUBLISHED MDA (WITH 2.1 DME) IS 680 FT MSL. I COMPLETED THE APCH; BREAKING OUT OF THE CLOUDS AT 700 FT MSL AND LANDED UNEVENTFULLY. I DEPARTED CMA. THE TWR WAS NOW OPEN AND THE ATIS WAS RPTING A REVISED MDA FOR THE VOR RWY 26 APCH OF 1100 FT MSL. IF THE 1100 FT MDA WAS IN EFFECT AT THE TIME OF MY APCH THEN I OBVIOUSLY WENT WELL BELOW IT TO THE PUBLISHED FIGURE OF 680 FT. THIS IS A POTENTIAL VIOLATION AND A SERIOUS SAFETY ISSUE FOR ME. I DO NOT KNOW WHAT ADDITIONAL STEPS I COULD HAVE TAKEN TO GET THE REVISED MDA INFO.