What does it take to convert a .40 229DAK from .40 to 9mm? thank You for looking

You will need a conversion barrel.

Barsto and the like will have them.

Or a complete 9mm slide assembly.

just the barrel or the slide also has to be replaced? the 9mm 229 mags will be needed also?

You only need a conversion barrel (not a factory barrel). Your 40 magazines will feed 9mm. You may need a 9mm recoil spring, but try without first.

Range use, yeah the conversion barrel would be ok with .40 mags.
If you are looking to carry it, I would look for a complete 9mm slide assembly (barrel, recoil spring, locking block?) and just use your .40 frame. Also get 9mm mags. I have a .40 cal no rail frame wearing a 228 upper assembly that has been totally reliable for the couple hundred rounds I've put through it.

Thank You for the info.

I have a Classic P229 DA/SA in .40/.357 Sig. I got a Bar-Sto 9mm semi-fit conversion barrel and had it finished out by the great Robert Burke. I use the same recoil spring, but did get 9mm magazines to ensure proper feeding and slide lockback.

It shoots great, and I now have three pistols that shoot all three calibers---my P229, my P320, and my P250.

I went both ways on my .40 model. I had a .357 barrel already for dual caliber use. A conversion barrel for fun and later a dedicated 9mm caliber exchange kit in 9mm when I decided to carry it off duty. In my case a locking block was necessary due to my German frame not being completely compatible with the US exchange kit. The 229-1 magazines make for a reliable weapon and I added a .22 exchange kit as well. All bases covered.