I am not familiar with the Bridge Code but I will attempt to answer your question on what I know about PQR's in general. A PQR is a record of what you did to a test plate, usually there are single values listed on the PQR. Such as the welder welded at a specific amperage, voltage, travel speed, preheat, etc.
Preheat would be determined by the welding process you used and the actual base metal thickness of the original test plate. The WPS is what gives you the ranges that the welder can stay within and still have a high degree of confidence that the weldment is okay. The WPS ranges would be determined by the referencing code (In your case D1.5).
Don't lose track of what preheat is trying to accomplish, to eliminate or control cracking. If the weldment thickness is vastly different than the original PQR then the original procedure may not gualify for the new thickness and conversely the preheat may not be adequate.
IF you have multipass welds to make then interpass temperatures will also play an important part and have to be considered.

If I can reword your question, I think you're asking that if the welding procedure was qualified for a low preheat temperature and D1.5 requires a preheat temperature that is higher, is it OK to use the lower temperature of the two? My answer is that the preheat temperature in the welding procedure is the minimum for ANY thickness of material, and the preheat specified in D1.5 is the minimum for the SPECIFIC material thickness being welded in production. You can not weld at preheat temperature that are lower than D1.5 specifies, regardless of whether the welding procedure was qualified with little (or no) preheat.

Marty