Chronic what 180 are you shooting now? If it is a quality spitzer (preferably a boat tail but that doesn't really matter) then I wouldn't bother changing. A mule deer is no harder to kill (from a physical aspect) than a whitetail. A 180grn wont be over 2"-3" lower at 400yds than an equally built 150 and they will still out penetrate the 150s as well.

I would suggest +3" @ 100yds and you will only have to put it atop the back at 300yds and about a 1.5' over the back if you are confident with a 400yd shot (bullet drop is easy, it's the wind that can be the kicker!!!). A couple boxes of ammo and an afternoon on a long range will give you all the practice and confidence (or prove your limit if shorter!) you need for your upcoming hunt. The main thing is you want to see what your rifle is doing in 100yd increments all the way too 400ish yds or whatever distance you quit grouping inside of 6-8 inches at. Write down that information and then put a small business card sized piece of laminated paper taped too your stock with that trajectory info on. That way you can see at a glance what you need to do. It is easy to forget those numbers when breathing heavy and staring at a nice trophy about to bound over the hill. (If you have the inclination and money a mil-dot scope would eliminate the need for the lil slip of paper!)

Good luck and above all have fun,
RA