Our main floor and staircase are dark oak hardwood, but I would like to tear up the wall-to-wall carpets upstairs and lay down a lighter floor, perhaps with a grey wash finish. Is this okay, or do I have to keep the flooring consistent?
You can't rip out those wall-to-wall carpets fast enough. For aesthetic and health reasons, hardwood with area carpets is the way to go. Hardwood gives you a clean slate. In terms of colour, a light grey matte finish is one of my favourites.
Ideally, a wood floor should be treated the same throughout the house, but unless you want to redo the main floor, celebrate the differences. The staircase could act as Switzerland and perhaps have black-painted treads and a white riser with a neutral carpet runner. Think of downstairs as a Republican (tough, strong but slightly old and dogmatic) and the upstairs a Democrat (fresh, but without a proven track record).
The debate is less about the different woods and more about the bad wall-to-wall carpet. My vote is for a third party: linen-trimmed sisal area carpets upstairs, with one foot of grey-tinted wood exposed.