This happens frequently and I have rebooked many times. With no change fees or award redeposit fees (as long as one doesn't book to BE), there's little to no risk to booking a fare if you think it's good enough but wondering if it could get lower. You can book it now and not have to worry if it goes up while still taking advantage if the price later drops. I do this for both revenue and mileage tickets. If on a revenue ticket, I'll happily take an e-credit because I have no worries that I won't eventually use it.

The one caveat is sometimes DL makes it difficult to rebook the same flight in the same cabin so you either have to call and have an agent do it or rebook to a different cabin or different flight and rebook back to your original flight and cabin, hoping that in the couple minutes it takes to do that, the fare you are truly rebooking to doesn't disappear.