The Lumia 820 offers the same performance and software as the Lumia 920 at a more affordable price, and you can also use it with gloves on. The phone is smaller and lighter, but we do miss the higher resolution for the display and the image stabilizer for the camera.
Any major computer upgrade generally requires a full software reinstall too. No so if you heed the advice in the guide to Upgrading a Motherboard without Reinstalling. PCSTATS Tips