LHR Premium Economy seats

I have booked my first Virgin Atlantic roundtrip (a Virgin virgin ;^)) between LHR from SFO through the travel agent used by my company. I purchased non-refundable premium economy and the travel agent assigned me seats 76A and 76H respectively stating that they were on the upper deck. After finding this site I see that these seats are actually in the middle row of the three premium economy rows on the main deck. I have never flown on the upper deck of a 747 and was looking forward to the experience.
What is the best way for me to procure upper deck seats? Call my travel agent and ask them to do so or ask when I check in at the airport?
Thanks,
Martin
What is the best way for me to procure upper deck seats? Call my travel agent and ask them to do so or ask when I check in at the airport?
Thanks,
Martin