I have always been careful to pay off my devices completely before selling or trading them in.
I may be worrying about this too much as I now understand that the phones may get re-flashed anyway.
I realize any installment plan obligation remains in force.

So a couple questions:
1. Do Carriers track a phones Installment Payment Status when trading the phone in? Does Apple or do they not really care? Did Carriers/Apple track the installment/lease status in the past?
2. Does it matter if the phone in carrier locked or unlocked?
3. Has anyone had a problem with a trade-in because it was under an installment agreement or a lease or carrier locked?
4. Do the phones get reflashed with new IMEI's when they are processed if they are newer models?

My main question here is to avoid a costly mistake when trading in phones using the trade-in programs, but also to avoid unecessary bother if current installment or lease status is no longer an issue.