B737 LNDG PHL GIVEN LAST MIN VISUAL APCH CLRNC CHANGE; BEGINS DSCNT PRIOR TO CLRNC.

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CHKED IN WITH PHL APCH CTL AND WAS TOLD TO EXPECT VISUAL TO RWY 27R. WE WERE ON VECTORS FOR THAT APCH WHEN ATC SAID TO INTERCEPT THE LIBERTY VISUAL FOR RWY 27L. IT TURNS OUT THAT WE WERE ALREADY ON THE MXE 108 DEG RADIAL AND WE DID NOT HAVE TIME TO ADEQUATELY BRIEF AND REVIEW THE 'LIBERTY' PROC. WE WERE ASKED BY ATC IF WE HAD THE TFC LINED UP ON RWY 27R AND WE REPLIED 'NEGATIVE.' IT WAS AT THIS POINT THAT I BEGAN TO REVIEW THE LIBERTY PROC AND APCH PLATE. IT STATES THAT YOU MUST BE AT OR ABOVE 3000 FT AT 33 DME AND AT OR BELOW 2100 FT AT 27 DME. WE WERE AT 31 DME AT THIS POINT; SO I STARTED DOWN TO MAKE 2100 FT OR BELOW BY 27 DME. GOING THROUGH 2700 FT; THE FO ASKED ATC IF WE WERE CLRED FOR THE APCH AND HE RESPONDED 'NEGATIVE; MAINTAIN 3000 FT; DO YOU HAVE THE RWY 27R TFC IN SIGHT?' WE QUICKLY RETURNED TO 3000 FT AND CALLED THE TFC IN SIGHT. WE WERE THEN CLRED FOR THE APCH. WE CROSSED THE 27 DME AT 2800 FT INSTEAD OF THE 2100 FT OR BELOW. WE CONTINUED THE APCH TO LNDG UNEVENTFULLY; MAINTAINING VISUAL CONTACT WITH THE TFC ON RWY 27R FINAL APCH. A LOT OF CONFUSION COULD HAVE BEEN PREVENTED WITH MORE HEADS UP NOTICE OF THE 'LIBERTY VISUAL' CLRNC. WE WERE CLRED TO INTERCEPT WITHOUT THE BENEFIT OF REVIEWING THE APCH PLATE.