As far as changing computers, you shouldn't worry as there's nothing stopping you from moving Mixcraft from one PC to another, even after the upgrade is released.
Now, unless I'm mistaken, only the standard Mixcraft is eligible for the free
upgrade (meaning if you buy v5 now, you get v6 when it is released).
For MC Studio Pro, I believe I remember Dan saying there was an additional fee (I think it was $50), to go from v5 to v6.
In any case, if you can buy now, you certainly don't lose out on anything.