Window Replacement (Inserts or Gut Out)
If you're thinking of changing materials then it would probably be better to replace the whole window, frame and all. I'm not sure how well fibreglass would slide against the (painted) wood.

If you just want to replace the moving parts then it's certainly possible, but you need to make sure that the thickness of the windows match. There are companies in the UK that offer a sash window renovation service - when I last checked some of them did offer this.