Well the agent said what I said about the risk to your reservation. It's just not the way that downgrading works, your reservation was fine, and wouldn't change, but to travel you need a ticket, and the ticketing process needs full and final payment to succeed. Once in a while someone mistypes a digit and the payment fails. But even if payment remains pending the reservation remains in place, possibly up to check-in, though I would certainly not advise leaving it that long, For a 27 March departure I would want that resolved by 24 March, so not that far off, and it's good that it is now resolved.