You can drain them, and then refill. You'll probably need two large pipe wrenches.
Be aware that the radiators are EXTREMELY heavy. And also, when you hook them up again, you'll have to be certain to avoid leaks.
Generally, it's better not to tamper with something that's working well. Assuming it's an old system, it can be difficult to get old plumbing to go back together without a lot of problems. There may be a way to paint them without removing them. Any paint store carries a special kind of roller with a long handle and a very narrow roller, which will reach down to paint the wall behind the radiator, or the radiator itself, as long as there's an inch or two of space between them.
So, to answer your question: yes, it's possible. But avoid it if you possibly can. It's a major job.
