Often1

If you look at US' COC, you will see that it is actually US' option to simply refund your tickets and be done with it. That's bad CS, so they've actually offered you the option of getting you closer.

You run the risk that US will simply decide to cancel & refund because it's becoming a hastle to deal with you, but you can certainly call back and propose what you are seeking (do a dummy booking to make sure that the seats you want are actually available and in te same fare bucket).

So, my suggestion is take the bird in hand, fly into TPA or run the risk of having your flights cancelled, refunded and then left to your own devices to rebook however you want. Only you can measure what that means.

But, I would not let this hang around. Your travel dates are around the holidays and you will soon enough run into a lack of cheap seats.