Buying PE now, upgrading to UC later

I've been giving conflicting information by two VS reps over the phone, so I thought I'd turn to the trusty V-Flyer gang for a more accurate answer!
There's great prices on PE flights to London from the US in December, but I'd prefer to fly UC if I can snag a deal (as I did last year).
So, my question is: If I purchase the PE flights now, and at a later date VS offers great prices in UC, will I be charged a charged a change fee for upgrading to UC on the same flight? Or simply pay the difference in fares? Yesterday I was told I only pay the fare difference, as long as I remain on the same flight. Today a different VS rep told me that I would have to pay $275 to upgrade to UC, in addition to the fare difference. So which is correct?
There's great prices on PE flights to London from the US in December, but I'd prefer to fly UC if I can snag a deal (as I did last year).
So, my question is: If I purchase the PE flights now, and at a later date VS offers great prices in UC, will I be charged a charged a change fee for upgrading to UC on the same flight? Or simply pay the difference in fares? Yesterday I was told I only pay the fare difference, as long as I remain on the same flight. Today a different VS rep told me that I would have to pay $275 to upgrade to UC, in addition to the fare difference. So which is correct?