I haven't flown BA for a long time, but when a company that I work for paid for my flight from LHR to DFW, of course I took that opportunity.

But I'm flying out from DFW back to LHR soon and I want to upgrade myself to at least WT+. Can I change my ticket on the way out? And how much would a upgrade cost at the counter at the airport?

Thanks for all your help!