well the first one is out of the paint shop now, so presumably will just sit about until they fix ther battery probs. The 767 fleet will be with Thomson until 2015 anyway, its just the planned redelivery dates which might have to to be pushed back.
The issue comes if they're not in service by 1st May 2013, since the B788 seats 33 pax more than the 763, and Mexico will have been sold at the higher number of pax, but again it's still only january so those flights are unlikely to be full yet so will possibly just be restricted to 258 on sale rather than 291.