Hi! I’m new here too, but I recently did something similar, so thought I’d share. I had old single-pane windows and didn’t want to replace the whole frame either. I ended up using Trident Glass Replacement Sydney, and they fitted double-glazed glass into my existing frames. It really improved the insulation and cut down outside noise, plus it was much cheaper than full replacements. Just make sure your frames are in good shape and get a proper measurement done. I’d definitely recommend them if you’re in Sydney—hope this helps!