Unless you bought a refundable fare wouldn't the difference be a JetBlue Travel Bank credit rather than to your CC? I'd still probably do it, but don't expect that you're getting a refund to the credit card.
As for fare range, it looks like JetBlue is only opening the $1339 inventory initially then eventually opening other fares. Some Saturdays in the near future get as low as $559 with the $759 and $959 scattered in. Though the lower fares are on less ideal flight times (very early/late).