Going 40-to-9 is not so bad, just a conversion barrel and maybe some 9 mm mags. Going 9-to-40 is more expensive because you would need the upper slide assembly and barrel, plus mags (for highest reliability). FN isn't selling the Compact slide assembly separately (last time I looked). But, there are deals to be had on complete new 40 compacts, at ~$340. A used one might be an option if you can find one??