Happened to me this morning EDI-LGW, 3 of us shifted from 1D, 1E and 1F to 10D, 10E and 10F, possibly the worst seats on a 737-400.

Of course our original row 1 seats which I was told were 'occupied' were empty until after take off when 1D and 1F were quickly taken by pax from the first few rows.

Needless to say I was delighted! E-mail has already gone to customer services.