First, why? What specifically will happen between copper and aluminum (besides just "fire")?
Second, I have also heard that the junction needs to be exposed to air, so a neighbor of mine has left his copper/aluminum wire junction box open and exposed inside one of his kitchen cabinets. I think he got "exposed" confused with "accessible," meaning that the box does not need to be exposed to air but also should not be, say, hidden behind drywall or something so that they can be serviced when necessary. Is that correct, or do these junctions need air exposure so as to not burst into flames? I would think that "exposed" would be more of a hazard.
Third, how necessary/useful would it be to undertake a complete rewiring and replace the aluminum wire with copper wherever possible? Is it something that I should not bother with, or is there a reason beyond mere efficiency that aluminum has completely fallen out of vogue? (safety...?)