Repair
Sorry will clarify - the way I understand it a number (12?) of F3 were damaged by incorrect repairs and so the stored F2 airframes were 'cut up' to get the required new structure centre section, patched in like a cut and shut to the F3.
Or did they gut the F2, bin the bits and engines and then take the F3 engines and kit and retro fit it to the earlier but largely untouched airframe?