Yes you can...there are two ways that i will share with you, old school and my way.
Old school says to sew the end of the new replacement lines to the old and pull them through.
My way is to cut a 12-16" piece of duct tape and lay it down sticky side up. Lay the end of your old reef line on it lengthwise halfway along one edge. Butt the end of the new reef line along the same edge of the other half of the tape and roll it up like a joint...er...uh...like a cigarette...yeah that's what I mean. This assembly should slide right on through the internal blocks, external blocks and clutch all the way from/to the cockpit. the best way to get the tape off is to cut it off by sliding a sharp knife , edge up, under the tape...if you try to pull it off, it constricts like Chinese finger cuffs.
It has been a while since I looked at the diagram in my owner's notebook, but I think there may be two separate lines