No such mechanism exists. Similar to how you can't exchange any TV bought from Curry's for the latest version - you own the TV regardless of how you paid for it. You can of course purchase a Gen 2 and try to sell your Gen 1. However, if there are still payments due for the Gen 1, this would still need paying in addition to the new payments for the Gen 2.
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Sky Glass 65” [Gen 1] with Sky Live
Sky Glass 43” [Gen 1]
Sky Stream Puck + Sky Soundbox
Sky Stream Puck x3
BT FTTP:
↓920Mbps
↑110Mbps
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