To convert any .40S&W to 9mm, you need as a minimum:
New slide (and internal parts) as the breechfaces are not the same,
New ejector (possibly)
Bottom line, it would be easier and cheaper to trade someone or to sell the 40 and buy a 9mm.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)