Will the caliber exchange kit work on a 9mm signs P229, or do you need to modify it? Thank you

Referenced Prouct: WCALX-229-357-BSS-10

That would depend on the P229 frame. If this is a newer P229-1 model that came with the 15rd 229-1 magazines, it would likely work. If it is an older frame P229 that came with 13rd magazines (this includes all German made frames), it is highly unlikely that it will work.