According to MSpoweruser, Microsoft will let users upgrade from Windows 10 S to Windows 10 Pro if they want. Windows 10 S is the new SKU from Microsoft aimed at the education customers. But just like Home users, for a fee, you can upgrade to Pro. While it takes $99.99 to upgrade to pro from Home but its going to take $49 or less to upgrade from S.
“Customers can easily switch to Windows 10 Pro at any time. For our EDU customers the switch is free of charge. For others, it will cost $49 or less to switch.”
However education users can upgrade for free till this year.