The last update for this question was a year ago in 2018 but can the option still be available to upgrade from 8.1 to win 10
From the 2nd paragraph of the article:
Update : The Assistive Technologies upgrade offer ended on January 16, 2018
Officially there are no more free Windows 10 upgrades available. Unofficially many retail Windows 7 & 8 license keys are still working for people. Plus any PC that tried Windows 10 and then rolled back is still considered licensed for the upgrade. Also you can run Windows 10 without activation with a few limitations.