had my 640 xl dual sim backed to 8.1 then upgrading to the latest 10549......scared me ! as it stayed in the 'black screen ' for quiet sometimes..... i was thinking the upgrade is failed.......
after staying in the black screen for ages ( 10 mins or more without any sign of progress), it finally displays ' almost done,...' the whole upgrade process is very slow compare to the last 10536 release. so if you have experienced the black screen, just wait for at least 10 mins or more. i think this version still has bug for upgrading. finally it updated. i dont feel the performance improvement at this moment. so i am going to do a hard reset and test again......
the 'migrating your data' process 1 through 9 takes some times to complete.
update for the 'hard reset': the phone is on the charger, then after reset, the phone stays long time in the black screen before powered up again. this also happens when i changed the language , need to re-boot the phone - it take long time to staying in the black screen before being able to powered up again.