Having been so critical of BA decisions on T5, this is a sensible decision. Give BA a chance. Let the T5 system bed in. I do not think it is there yet.
It will be interesting to see if BA goes for another big bang (would be nervous if this is the case) or if the migration are staggered. BA do need to ramp up staffing levels over 2/3 weeks but surely it cannot cost more than all the compensation and lost business that has hit it to date.