Unfortunately, usually you need a key to turn the lock in order to seperate the lock stem from the door latch mechanism.

The alternatives are not pretty or easy, you might be able to unbolt the door latch mechanism enough to manipulate it enough to line up the lock stem so as to allow it to be removed. An alternative would be to cut the lock stem allowing the existing door handle/lock to be removed, then you could install a new stem on the existing door lock handle. If you're swapping the existing door handle/locks for newer ones with keys then once you get the old ones out just put the new replacements into their place.