You can put a .45 FCU in a 9/40/357 kit as many have done before. To be clear, you need everything, grip module, mag, slide, barrel, rsa.. Bear in mind that the takedown safety (blocks takedown with a mag still inside) will no longer function.
The .45 ejector is a bit shorter so ejection may be affected (although others say it's fine). The slide lock lever is also different but I don't recall that being an issue.
Sig still has some xchange kits for 9, .40 and .357sig on their website. Not many left, so you should grab one while you can. .40 to .357 sig (or the reverse) only requires a barrel. The mags are the same. The grip module is the same for all 3 but you need a complete top end and mag.