Replacing modern double-hung windows - options?

My new home isn't so new any more. My Andersen double-hung windows installed in 2000 are starting to have issues. I have some rotting lower sashes - cause unknown. Some have failing and hardening weather stripping that leads to more air infiltration. And from the beginning they were never as "tight" as i was hoping against the stiff winds we sometimes get. What are options for window replacement? Is the true only option to remove the entire window unit the same way it went in which requires messing with trim and such on both the inside and outside of the house? I really need to address some rotting lower sash rails this coming year and the cost for a sash approaches half the cost of a new window (parts only).

Anyone have any wisdom to offer?
THX> Eric

 

Your windows are likely new construction, (with nailing fins) and so they need to be replaced with new construction windows of the same size.