I have a Daytona tactical upper and just ordered a BCM KMR-A to replace the hand guard that it came with.  The Daytona tactical came with a cheap keymod rail.  My question is, will I be able to simply remove and replace the hand guards or will I need to replace the barrel nut?  If I need to replace the barrel nut, that means I need to remove the comp, gas block, etc, and I'll need a new crush washer?  Anything else other than the tools?

In Short.... YES

The barrel nut will need to be changed.  The Barrel Nut ID is not big enough to fit over the gas block, the gas block is not big enough to fit over the comp.  If your using a crush washer you will need to replace the crush washer on the comp.  Since your going to have to remove and reinstall it all get a set of accuwashers.  

Precision Armament Accu-Washer Muzzle Device Alignment System

1 set is good for several builds (I have 3 out of my set so far) and they are re-usable.

Yes, you'll have to switch to the BCM barrel nut. And use a new crush washer if you have one. Make sure you properly align the gas block when reinstalling it. Easy job with the right tools.