The socket isn't going to be an issue, as Haswell and Broadwell both use the LGA 1150 socket. You do need to be concerned about what CPUs your motherboard supports, however, as certain Intel 8X series chipsets won't work with Broadwell.
You probably want to stay with an i7-4790k, because Broadwell CPUs aren't widely available and they are more expensive. The iGPU inside of it is much better, but CPU performance isn't really any better overall and you can buy a graphics card with more performance if you really need it.
aaron3238 did a good job of explaining the process of replacing the CPU, just one small thing to mention. If you have an aftermarket cooler, you will want to purchase some decent thermal paste also and re-use that cooler. If not, then you will likely want to order one to avoid heat issues. Intel's CPU coolers aren't particularly effective at cooling the Core i7 CPUs.