Originally Posted by
findark
Yes. Upgrades don't change the value of your ticket coupon.
Correct, it does not. Normally an upgrade isn't actually based on the price of either cabin directly, but you may be seeing a special case or you may be looking at "change flights". Especially if the latter, then yes you need to pay the difference from Main since that's what you purchased, and the free upgrade has no bearing on that.
Thank you so much for the information! These are very helpful