When this house was redone some years ago, we took the old frames w/ the
sash cords out and replaced them w/ new track systems. The windows
themselves were either refurbished if in decent shape or the original
glass fitted into new frames (even the lower panes are old enough to
show the imperfections of glass from that time so to lose that would be
to change the "patina" of the house dramatically. We then added storm
windows of an appropriate style for the enhanced energy savings.

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That's an interesting approach. How did you get it done? Is it
something most contractors can do?

Regarding keeping the character of the house goes, all of our trim and
moldings are very dark so we would need something similar in the
windows. We have some friends with a similar style that stuck in white
replacements, and they look terrible against the wood.