Difficulty of Switching NSF GFRP Subfield

[moosetiger]

Incoming statistics PhD student (awarded NSF in statistics) who has recently realized that their interests lie more in applied math (PDEs, mathematical biology and a couple other subfields). If I continue with my current statistics program, however, I will be unable to do research in applied math (since my program’s department requires the dissertation to be highly statistical in nature): thus, I am hoping to apply to my current institution’s applied math program and transfer my NSF award to the “applied math” subfield after the first year. 

Does anyone have any experience with transferring subfields after the first year, ie how difficult it is? I am torn between either attempting to transfer departments or applying to a new program entirely. If transferring NSF subfields is almost impossible, though, would really appreciate knowing so can best guide decision…