I don't know about IDPA, but in USPSA Production, the gun has to be as produced by the manufacturer. Changing the caliber from .40 to 9mm makes it illegal in prduction.
Here are the allowed mods.
Authorized modifications
(Strictly limited to these
items and their stated
guidelines)
•Internal throating and polishing to improve accuracy,
reliability and function
•Sights – trimmed, adjusted, replaced, colored, or
fiber-optic.
•Slide – refinishing. Milling of slide – only as
required to insert sights.
•After-market slides and barrels – provided they are
the same length, contour, and caliber as original factory
standard.
•Grips – Internal beveling. Checkering, stippling, and
addition of grip tape or grip sleeves. (see Appendix
E4)
•Exchange of minor components (springs, safeties,
slide stops, guide rods).
Hope that helps clear it up.