The card was changed (upgraded). Any port change including a card replacement would change IP.
Similar, you were moved to a different port.
Bonded pairs need two ports on same card, it is possible to provide a bonded service your service was moved to a different card freely up a port on the original card.
Software update... While generally gateway swaps and software updates do not change IPs, it does not mean it cannot happen. Thus a different gateway installed or a software update pushed out could change IP.
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