Old oil become contaminated from the by products of combustion. Get it hot enough and it begins to form an acid that can cause corrosion. Ever disassemble an old engine and find orange staining? That from the acidic effects of old oil and sitting.
Changing the oil should reduce the amount of this acidic oil. Change the oil, run engine for a short time and the old oil should be flushed/diluted before storage.
With that said...I sometimes leave old oil in there.
True as that may be, there are brand new cars that sit on car lots the amount of time that a car might be in storage over winter. I don't think it's the end of the world to say the least. It may be worse if you have really old used oil in there before you put it away.