[Zeke]

Another silly question, how does ducting get "worn?" But, yes, as long as the sizing is close to the amount of area used for the existing, the flow will remain the same. You can wrap some of it with the rolled fiberglass lengths to save tearing it apart.

If it's poorly insulated or just rigid bare duct, do something. The flexible stuff is nice with a sanitary liner and an exterior sheath over the fiberglass insulation. That old fiber board interior/metal exterior is nasty.