Quick compatibility check.

can I slap a P229 9mm complete upper on a P229 .40 frame and have it run? Looking to shoot some 9mm out of my 229 .40 and have a bead on a complete 9mm P229 upper. Are these interchangeable? Thx

Depends on which generation 229 in 40, you may need to switch out the locking insert. Not very hard to do.

A 9mm P229-1 upper will drop on a .40 frame and work.

A 9mm P229 upper will require you to change the locking block in the .40 frame before it will work.

The 9mm barrels are different between the P229-1 and original P229. The original P229 9mm used the P228 frame and barrel (and the same 13 round magazines). The newer P229-1 9mm was designed to use the P229 .40 caliber frame so the new 9mm barrel mates with the .40 locking block while the old 9mm barrel will not. Besides the locking block area the barrel hoods (and thus the breech face on the slides) are different between the P229 and P229-1 9mm's.