Pistons have coolant ***help***

Yes do an oil change before you crank over the engine. That way, as Pistha mentioned, you won't pull the coolant from under the oil in the pan into the engine. For that matter I would just pull the drain plug now and let it drip till you have the engine back together.

As long as the level of the coolant is below the areas you are working on you don't need to remove anymore from the block.

Are you going to get it all back together in a few days or months? if days just get er done. if months I would make sure the cyl walls are coated with an oil type substance, engine oil or other spray oil, (probably not penetrating oil)