This actually isn't the true situation. The first phase of the TP is a massive new-build extension of T2 out to the West and onto 'Staff Wales' Car Park. Only when this terminal capacity comes online will there be a need to withdraw (much smaller) chunks of the existing complex section by section. Note that the TP increases T2 passenger capacity by 140%, so don't underestimate the scale of new-build involved in the extension to the West.
I hope you're right as well SHED. My reading of the (admittedly limited) designs released to date is that whilst there is an extension to the current 'core' terminal building to the west and north, the largest element of the capacity increase will be the addition of the new piers. Certainly as the work is undertaken to add the middle and eastern piers - which looks like a new airside frontage being required too - capacity will drop, potentially by some margin.
Don't forget that it is not just space for passengers affected by the work but for aircraft too. Most of the midfield remote stands will be impacted by this work and I suspect that Pier C of Terminal 1 will have to go before the eastern pier of T2 can be built.
I would be interested in views on which carriers will go where in the post expansion world. Whilst alot depends on how Pier B is integrated into the T2 layout I can't see how, for instance, T3 and Pier B alone could accommodate EZY, RYR, LS and BEE, yet I don't see there being space for them at T2 either?