You have to disassemble the forks prof, to change the oil.To loosen that Allen bolt, you have to push up, to compress the fork a bit,the spring tension will cause resistance so it wont spin.And you should be able to crack it loose .If I remember correctly, that bolt secures the piston. The oil Will drain when you remove it. And no, unfortunately, you can't just remove the top caps and fill. It's a big,time consuming job. Have your service Manual handy, and set aside enough time. Choose your fork oil wisely, you wouldn't want do do it twice.