I bought the onslaught Map and after that I saw that the season pass included it, so is there any way to upgrade from onslaught to Season Pass at a lower price?
Short answer : No.
If you buy the Season Pass it will cost $50. Buying the remaining map packs separately will cost you $45.
So you end up paying 5 bucks more. However you will get the patch, background, reticle, etc that comes with the Season Pass. And all map packs will be instantly ready to download when they are released. Your choice