Long ago I wanted to tune lower to accommodate my singing voice so I put a heavy gauge set on a dreadnought style guitar and went to about
.060 on the low E and then tuned down two whole steps to C and sometimes to B or Bb and it worked out fine but proved not to be very versatile as I only had one guitar at the time. If you do this you'll want to look at a string tension chart, they're available online, so as not to over string it or better yet call a luthier and see what he or she thinks. Plus bear in mind you might have to wallow out the nut slots to get the strings properly seated which is something you should only so if you're comfortable with it.