AN ACD DC10-30 FLC HAS AN NMAC WITH VFR TFC THAT APPEARS TO BE CLBING. FLC FOLLOWS THEIR RA AND CLRS THE TFC AND CONTINUES TO SEA.

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CLRED TO CROSS 'JAKSN' INTXN ON GLASR ARR AT SEA-TAC. WHILE DSNDING THROUGH 13500 FT; WE WERE ISSUED A POSSIBLE VFR TARGET AT 11 O'CLOCK; 12500 FT AND 2 MI OUT. I BEGAN TO LEVEL OFF AT 13000 FT AS A PRECAUTION; WHILE WE LOOKED FOR THE ACFT. TCASII SUDDENLY GAVE US AN RA TO 'CLB NOW;' WHICH WE COMPLIED WITH. SAW TARGET ACFT AT SAME INSTANT RIGHT BELOW AND JUST AHEAD OF US. VERT SEPARATION ACCORDING TO TCASII WAS 400 FT. DURING CLB; AT 14000 FT; TCASII ISSUED 'CLR OF CONFLICT.' NO FURTHER PROBS. THE ENTIRE OCCURRENCE; START TO FINISH WAS PROBABLY LESS THAN 2 MINS. THE ORIGINAL 'HEADS-UP' CALL OF POSSIBLE TFC BY THE CTLR ALLOWED ME TO START SLOWING MY DSCNT RATE AND LEVEL AT 13000 FT; WHILE LOOKING OUTSIDE FOR TFC. RA FROM TCASII WAS JUST AS ADVERTISED; AND IN THIS CASE PROBABLY PREVENTED A MIDAIR. SEA-TAC ARPT HAD EARLIER BEEN LNDG TO S; RWY 16R; WHICH REQUIRES A 'JAKSN' XING OF 12000 FT; BUT HAD SWITCHED RWYS TO LNDG N; RWY 34R; WHICH REQUIRES A 16000 FT XING. CTLR WASN'T AWARE OF THE RWY CHANGE; AND WAS STILL ISSUING THE 12000 FT ALT TO ARRS.