I have a building with a dark brown stain and want to change it to dark sage green color with cream on the eves. My questions are: How do I know if the original stain is oil? Do I need to use a solid stain to cover the dark brown? Should I use a oil solid stain? Should I pressure wash?
Without the original can of stain it is difficult if not impossible to tell if stain is oil based or acrylic. It might not matter depending on the condition of the original stain.
First lets talk about prep. Yes you will need to pressure wash. This removes old oxidized stain and helps open up the wood pores to accept the new stain. You should use a siding stain stripper to remove even more stain if the current stain is in great shape, you just want to change color. A nice stripper to consider is Gemini Safe Strip, it can be injected through the pressure washer and is environmentally friendly.
Usually a good careful washing is all that is needed. Painted surfaces that are peeling are prepped the usual way.
You will need to use a solid colored stain to fully cover the old stain unless it all comes off. That is very doubtful. Personally I like the oil stains for their penetrating ability plus they stick to everything. You could use an acrylic instead as long as the washing is very complete and sufficiently opens up the wood pores.