It would be much easier to buy a new barreled upper than swap out A2 to A4 receivers. You could sell the mixalot upper instead. And get the barrel/profile/gas length/twist you really want along with the A4 upper.
And you'd know exactly what you have!
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek