I just noticed this in the travel re-accommodations section of CO.com:
Original Travel Dates: Feb. 27, 2008 - May 15, 2008
Change fees and/or additional fare collection will be waived for chages from London, England (LGW - Gatwick) to London, England (LHR - Heathrow) booked in the original class of service on the same travel dates as originally ticketed.
If the new travel is in a different class of service, change fees will be waived, however, additional fare collection will apply.
Why is the start date listed as Feb. 27? Service on CO metal doesn't start until March 29. I wouldn't expect them to let you switch from the CO metal to the VS metal flights. I wonder how LHR bookings are looking.