I already own a P228 and I'm wondering if the holsters and such will still work for a P229.
A lot of this may depend on the holster maker. Some may, some may not.
Is there a difference in the slide between 9mm and the 40S&W?
There is with the P239's, not sure with the P229's. My P239 is a 357SIG, and all of the holsters I've bought for it, have been caliber specific..
One issue between the P228's and the newer P229's is the rail. The older P229's dont have one. Newer P229's with the rail wont fit the older P229 holsters. Some makers make holsters for both, but some dont, or at least they dont seem to differentiate.
These days, I buy my holsters for the guns with the rails, that way, my older guns will also fit. The difference this way is hardly noticeable. I do have older holsters for guns that dont have rails, and the rail guns wont fit at all, or are very tight.
As far as using the same holster for different models, my P220's, P245, P226's, and P6's, will all use the same (rail model) holsters. They will also work in my P229 holsters, as will my P239, but they are a sloppy fit.