This is an easy thing, even if the existing line is in the furler. If you use a painter of sufficient length, you butt the painter and halyard ends together, attach, and pull (carefully) the new painter through, with the old halyard.
Then you remove the old halyard, attach the new one, and use the painter to pull the new halyard through.
The hardest part is probably paying attention to the joint between the painter and the halyards.