I have Andersen Narrowline Double hung windows from the 70’s, what I can replace them with?

You have three solutions; first Andersen makes a Tilt Wash Conversion Kit to transform your current window into a modern day 400 series Tilt Wash without the need to remove the existing frame or install new interior trim and siding outside. Second a new replacement window can be installed inside the existing frame. Third a full frame window can be installed to completely replace the window down to the rough opening. This method is the most expensive and invasive but it is the best way to insure 100% air and water sealing and integration with the water management system.