A BE58 INSTRUCTOR AND STUDENT APCHED APC ON A VOR 18 VMC. RWY 24 BECAME ACTIVE CAUSING A CLOSE IN RWY CHANGE AND A POTENTIAL CONFLICT WITH VFR TFC.

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MY STUDENT WAS FLYING THE VOR RWY 6 APCH INTO NAPA COUNTY. WE WERE ON AN IFR FLT PLAN AND CLRED FOR THE APCH IN VMC. THE ATIS INFO WE HAD STATED THAT RWYS 18L&R WERE IN USE. THE ATIS WAS ABOUT 1 HR OLD. I THOUGHT ABOUT UPDATING THE ATIS; BUT ZOA WAS GIVING US NUMEROUS ADVISORIES FOR TFC. CTR HANDED US TO THE TWR. WE CONTACTED NAPA TWR; WHO ASKED US TO RPT THE SGD VOR INBOUND. THE TWR CTLR HAD ALSO MADE A COMMENT THAT HIS RADAR WAS INOP. WHILE XING THE VOR; I WAS LOOKING TO OUR L FOR TFC THAT WAS INBOUND FROM THE NW; WHILE SIMULTANEOUSLY; THE TWR ASKED A BONANZA IF THEY WERE NEAR THE SGD VOR. THE BONANZA PLT SAID HE WASN'T FAMILIAR WITH THE AREA AND ADMITTED TO BEING A LITTLE LOST. I LOOKED TO MY R AND SAW THE BONANZA ABOUT 1500-2000 FT DIRECTLY ABEAM US. I THEN TOLD THE TWR THAT I SAW THIS TFC IMMEDIATELY TO MY R; BUT I DIDN'T SPECIFICALLY STATE THAT WE WERE AT THE VOR INBOUND. BY NOW; WE WERE APCHING CIRCLING MINIMUMS AND THE TWR FREQ WAS SUDDENLY BUSY WITH THE TWR TALKING TO THE SEEMINGLY LOST BONANZA PLT. WE STARTED TO CIRCLE TO THE W FOR R TFC RWY 18R; WHICH IS THE NORMAL PROC FOR RWY 18R. WE STILL DIDN'T KNOW THAT THE TWR HAD CHANGED THE ACTIVE RWY TO RWY 24 ON THE ONLY MINS OLD NEWEST ATIS. THE TWR ASKED US WHERE WE WERE; AND I SAID WE HAD STARTED CIRCLING FOR R TFC. HE REPLIED WITH; 'ROGER; CONTINUE.' I SUDDENLY REALIZED THAT THE NEW ACTIVE RWY WAS RWY 24; BUT WITH THE CTLR'S RESPONSE AS; 'ROGER; CONTINUE' I ASSUMED THAT HE SAW ME AND DIDN'T HAVE A PROB WITH MY MANEUVER. I CLBED TO NORMAL PATTERN ALT AND WAS CLRED TO LAND RWY 24. ONCE ON THE GND; I SPOKE TO THE CTLR AND WE TALKED ABOUT THE SITUATION. I BELIEVE WE REACHED A MUTUAL UNDERSTANDING AND HE SAID THAT THERE WAS 'NO PROB.' REFLECTING BACK; I SHOULD HAVE MADE A MORE SPECIFIC RPT WHEN XING SGD VOR; BECAUSE THAT IS WHERE THE TWR USUALLY ISSUES CIRCLING INSTRUCTIONS. I SHOULD NOT HAVE ASSUMED THAT THE ACTIVE RWY WAS STILL RWY 18. HOWEVER; EVEN IF THE TWR TOLD ME TO CIRCLE S FOR L TFC RWY 24; I DON'T THINK THAT IT WOULD HAVE BEEN SAFE TO DO SO; BECAUSE THAT WOULD HAVE REQUIRED TURNING TOWARD THE LOST BONANZA. THE BEST THING TO DO WOULD HAVE BEEN TO MAKE A MISSED APCH; CANCEL IFR; UPDATE MY ATIS THEN RE-ENTER THE PATTERN VFR.