When you are upgrading an itinerary but not changing flights (ie: going from Tango Plus to Latitude) is the change based on the original price I paid for the ticket?

IE: Tango plus when I reserved it was $500. Today, tango plus is $700.
Latitude today is $1000.

If I want to upgrade fares, does it take the current value of my reservation ($700) or does it take what I actually paid ($500)?


thanks