I know that recently on my CLT_LGW leg when I had left the transatlantic upgrade certs on my desk at home (*gasp*), the agent first asked if I would like to upgrade w/ miles - until he noticed I was booked in K class, which couldn't be upgraded w/ miles.. so I think you can, but it's risky... if you're prepared to do that, why not go ahead and do it now? Hoping for an operational upgrade?