It’s definitely possible to change out the hinges without redoing the cabinets – you would just have to make sure that the holes on the frames where the old hinges were, are able to be filled and touched up so that they’re not obvious. I’m not sure of the cost, but hinges aren’t cheap, and I definitely recommend using a carpenter to do the work. Hinges can be maddening!
When I’m talking about metals, I consider the silver metals as one grouping. So, when you have stainless appliances (silver), a chrome faucet (silver) and brushed nickel lighting (silver) those all count as one metal in my book- SILVER. I personally think Nickel is the most timeless of the metals. Brass has really gained in popularity with interior designers, but is probably a trendier choice. I’ve seen a rise in the popularity of black hardware especially in this age of farmhouse- EVERYTHING (Thanks Chip and Joanna). I think every room in your house should have a little black in it, so if you can bring in the black with your second metal in your kitchen, DO IT!!