I believe you meant on Windows 8.1. No one is forcing you to update to Windows 10 Mobile. The only way your phone can be updated to Windows 10 Mobile it's if you do it manually using Insider app it Update Advisor app, it won't happen on it own, so no worry about that.
Thank you. It's shocking how much better wp8.1 is. No crashing, just no problems. A phone where all features just plain work. Guess I forgot after trying w10m AU.