The 'old plane' is having its radome replaced and is due to operate the BA245 to EZE tonight (G-YMMD).
Tonight's BA9 is now being operated by G-YMMB which had just arrived from BKK and was turned around as quickly as possible. This aircraft was originally meant to operate the BA245 so in essence it is a 1-4-1 swap. It does mean that in all probability, tomorrow's BA245 will also be delayed as the BKK flight tonight was late leaving LHR.
There isn't much slack in the 3-class 777 fleet at the moment. LGW have 2 aircraft based there (1 was AOG yesterday) and G-YMMF is in maintenance at Cardiff. There aren't many opportunities for tail swaps if there is a technical problem.
Hope they enjoy the flight nonetheless!
Champ