Phone Upgrade

I currently have the Go5G Plus plan that lets me upgrade every two years. When I look on the app it says that the trade in for the new iPhone is via 36 months of bill credits. Does this mean that the device wouldn’t be paid off for 3 years instead of my plans 2 year upgrade? Would I essentially have to pay the remaining credits to upgrade in two years? This doesn’t make much sense to me since my plan says upgrade ready in two years so you’d think I’d be able to get full credit of the device at that point.