ADVISE ON UPGRADING TO BUILD 10572 - I had problems upgrading and still had issues until I read somewhere that taking these steps makes Build 10572 work smoothly, so I tried and IT REALLY WORKS!!
WARNING - This is a LOOOOOONNNGG process, but VERY MUCH worth it!!
Follow these steps and Build 10572 works GREAT!
1) Backup any data you don't want to lose to your PC or the cloud (pictures, documents, videos, etc.)
2) If you are on Win10 build - return your phone to Win 8.1, DO NOT RESTORE!! If you are on W8.1, reset your phone to factory settings - DO NOT RESTORE.
3) With phone now on Win8.1 factory settings, run through the setup process, log in and immediately download the Windows Insider Preview app. You will have to wait for all the other apps to be updated as well.
4) After all apps are updated and Windows Insider Preview app is installed, open Windows Insider Preview app and choose "Fast Ring"
5) Phone should restart and download and install Build 10572.
6) Again, run setup, log in, and allow phone to run updates for all apps - DO NOT RESTORE!!!
7) RESET your phone again! Yes, you read that correctly - RESET Win10 Build 10572 to factory settings.
8) Reinstall the Windows Insider Preview app, open it and choose "Fast Ring"
9) Phone will restart and install Insider Preview edition.
10) At this point, you should have Windows 10 Build 10572 Insider Preview version reset to "factory settings." You can now begin reloading all your backup files and manually install any apps you want on your phone.
I did these steps and my Lumia Icon is actually running better than Windows Phone 8.1 did. Battery is amazing now! The only thing I don't have at this point is "Hey Cortana," but I'm sure we'll get that in future builds. I was a bit skeptical and don't exactly know why this works, but believe me, my Lumia Icon is running like it was designed for Windows 10! The back of the phone is actually COLD to the touch. I only lost 3% battery in 45 minutes and I was on a call for 20 of those minutes. Screen is smooth and quick. Apps launch quickly and have not had any crashes since doing this. Camera had a slight instance of delay in "processing" photo, but that has since disappeared. I'm finally ready to leave this build on my Lumia Icon and use as a daily driver. I just need MS to fix the "Hey Cortana" thing now, but I'm EXTREMELY please with how my Icon is working now!