B757 FLT CREW HAS A RWY INCURSION WHEN A RWY CHANGE IS MADE WHILE THE PIC IS ON THE PA WELL SHORT OF THE HOLD LINE FOR RWYS 8 AND 12 AT MIA.

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TAXI TO MIA RWY 8R. WHEN APPROX 6000 FT SHORT OF HOLD SHORT LINE; NOTICED WE WERE #1. AT 3000 FT SHORT OF HOLD SHORT; COMPLETED PA; CAME BACK ON RADIO WITH FO; HEARD 'CLRED ON TO HOLD; KEEP IT ROLLING; CLRED FOR TKOF; TFC 4 MI FINAL; TURN R 120 DEGS AFTER TKOF.' TURNED R TOWARD RWY 8R; CROSSED RWY 8R HOLD LINE. TWR INSTRUCTED CONTINUED R TURN AND CLR RWY. DID NOT HEAR TWR CHANGE RWY TO MIA RWY 12.SUPPLEMENTAL INFO FROM ACN 636236: WAS TAXIING DOWN TXWY M AND WAS EXPECTING TO DEPART RWY 8R. TOWARDS (NOT AT THE END) THE END OF THE TXWY AREA AND PAD AREA; ATC CHANGED THE RWY TO RWY 12 AND INFORMED OUR FLT TO KEEP THE ACFT ROLLING AND GO TO POS AND HOLD. THE FO ACCEPTED THE CLRNC AND CHANGED THE HDG SELECTOR FROM 92 DEGS TO 123 DEGS. ALSO THE RWY WAS CHANGED IN THE CDU FROM RWY 8R TO RWY 12. THE FO ASKED THE CAPT IF HE WAS READY TO GO TO POS AND HOLD ON RWY 12. THE CAPT SAID HE WAS READY. AT THE END OF THE RWY ENVIRONMENT; THE ACFT MADE A VERY ABRUPT TURN TO THE R AND WAS NOW CREATING AN INCURSION ONTO RWY 8R. THERE WAS VERY LITTLE TIME TO REACT TO THE INCORRECT TURN AND IT SEEMED PRUDENT BY THEN TO KEEP THE TURN GOING RIGHT ROUND TO 180 DEGS TO REMOVE THE ACFT FROM THE RWY 8R ENVIRONMENT ASAP. ATC WAS AWARE OF THIS SIT AND RECLRED FLT TO POS AND HOLD ON RWY 12. FLT DEPARTED RWY 12 WITHOUT ANY FURTHER DELAY.