Second on the complete 9mm slide. I bought both the P226 and P229 RX slides when they were on sale last year. Only thing I needed extra was a handful of 229-1 mags.I bought a CPO 229 in 40 which was totally refinished and looked brand new, came with SRT and all new springs. Couldn't find one flaw on it. Shoots the 40 like a champ. I can tell it was designed for the 40 cartridge. I love it. Also have an early 229 in 9mm which is nice.
You can keep the 40, buy a 9mm conversion barrel or even a 229 slide in 9mm for it if you wanted to go that route.
40 is a really versatile cartridge, outside of factory ammo you can load it weak to hot. What is now considered hot was once the original load.
I vote keep it!