I don't think the overall scale in difficulty is changing (excluding the nerf to monster damage across the board), but the drop rates are going to be the same across all 4 acts. That's what I gather at least.
No difficulty change.
And no drop rates change UNLESS you activate Monster Power (lvl 1 minimun)
If you have MP 1 (or more) then all monsters will be lvl 63 (with no MP, act 3 and act 4 monsters are lvl 63, that is why you will get the same drop rates that you have in act 3 and act 4 right now)
MP makes the difficult higher, bit relative to the act, since it buff the monsters of the act, so, yeah, act 1 MP 1 (or more) should be easier than act 3 MP 1 (or more), and you have the same drop rates everywhere as long as MP is at least 1
Hope this helps you