I began by hard resetting my original att lumia 1020 us on 8.1. Then I used the new Windows Mobile Desktop Recovery tool to make sure firmware was full denim and it was. Then hard reset. Installed Insider and got slow ring. some how got lucky and went to Denim GDR 2 performance was insanely amazing best ever, although battery drained rapidly at times. Then I hard reset no restore. got build 10166. Then I updated all lumia apps and then changed insider to fast and got build 10512. Let me say performance was aaaamazing, on par and crisper larger graphics then 8.1, and the numbers from my phone app were equal to denim gdr2. But even after all this no lumia apps? So I hard reset on build 10512. Boom all lumia apps and 8.1 apps ready to go. Yes. Not Yet? performance was horrid? SO hard reset and luckily I had saved a backup of my first build 10512 and its performance and what did before my wondering eyes had appeared all lumia apps and performance numbers on par with 8.1, and 8.1 apps working fine, and all new 10 experience and custom thingys lol!! If I'm not forced to I want this awesomeness forever!!