It can always downgrade if you try to upgrade in one go multiple times. Here is a small calculator I made so you can enter the Nightmare you're on and see what will be min/max modifier you will get at that stage if you press upgrade button :)
https://jsfiddle.net/MartinTale/fb9hsk1v/show
Ooooooooh I understand now! It's not upgrading your existing multiplier, it rerolls it completely ^^
Thank you for the explanation!
No worries :)
