I am considering getting a 5.45x39 upper to put on my lower and purchasing 1000 rounds of [corrosive] russian ammo. It ends up costing about .10 per round instead of .30 which is the cheapest I can get the 5.56.

Has anyone done this? My only concern is what the "corrosive" ammo could do? Do I need to swap out any parts on my lower? I'm not completely sold on the idea yet.

You might consider the nickel boron fire control group. You'll need a heavier hammer spring, which the upper comes with. Corrosive 5.45 is not an issue with the Spikes upper.