Welcome to Justanswer.com, my name is ***** ***** I will do my best to assist you.
May I ask why you didnt install the windows update directly on the windows 8 computer, using the Windows update feature?
Also, as far as the USB stick goes, how exactly did you create the USB stick?
It shouldnt be an issue upgrading from WIndows 8 to Windows 19.. Even windows 7 can upgrade to windows 10 without too much fuss.
Question, when you say the windows 10 update wasnt available, do you mean that the Windows 10 update button said your download / upgrade wasnt available yet?
ALso, can the computer currently start and boot into windows 8 normally?
Im not sure exactly what you mean when you say you do not want to switch..
Can you please be a bit more detailed and specific when you reply, thank you~