The only reason why they replace the top case along with the battery on the retina models is because the battery's glued in place. As for the non-retina models, it's just a slightly exotic tri-wing screw holding it in place and those screwdrivers can be easily found on the internet for as little as $5.
If you don't need to keep the battery, then you can just was well get a metal spatula and pry it off. Hell, if you're careful you can even get it off without any damage to it.
In other words: Apple has made battery replacement a pain, but you can still do it yourself without having to replace any other components.