If it were me, I'd replace the whole thing. At the very least, I'd get
quotes on what the cost is to replace the whole thing now, versus what
it will cost if you want it done seperately. I think you'll find it
will cost considerably more to do it piece meal and at 23 yrs, you're
on borrowed time. Also make sure to look into any rebates from the
elec company that are frequently available.