There's a right way and there's a wrong way. You may get away without converting the frame rate but your results will be unpredictable AND the quality of the frame rate adjustment that FCP will make with this workflow will probably not look as good as doing this properly in compressor. If you decide to go the compressor route, be sure and set the retiming to best in frame controls.
Replacing the footage after conversion should not be that difficult. eyematching and using the gang function to confirm that you've matched the shot correctly goes pretty quickly.
another thought. You could keep editing with the mixed frame rates and then file: export: quicktime movie with current settings and bring that file back into fcp and just replace any shots that you feel aren't up to snuff.