So, tell me if this is possible. I own two Sig P226s... one standard and the other an M25, both in 9mm is it possible to convert either to Sig 357 and if so what would I need?

I believe you would need a new slide with barrel assembly. A barrel probably won't fit. The 357 barrels are usually sized larger than 9mm barrels.

Yes a new slide assembly is required. Going from 357 to 9mm a barrel and recoil spring is required.

You need a 40 slide and a 357Sig barrel. The ejector is slightly different also. The breech face cut on the 9mm slide isn't wide enough for the 40 cal casing from what I remember. The barrel hood widths are different between 9mm and 40/357.
Joe

For the price of the exchange kit you'd be almost there on a complete 40S&W P226, just saying...plus those are in stock.

P226 DA/SA Police Trade-in

The barrel would be additional but then you'll have a dedicated gun for 40/357 instead of swapping things around.

Thanks to all for your replies....

So, if I bought a P226in 40 Cal and got a 357 barrel, would I also need a different recoil spring. And it sounds like the 40 cal mags would work?

The 40 and 357 use the same recoil spring and mags.