Check the C&C Canon thread in the Tiberian universe section. But regardless it's actually a plausible transition fact-wise if you look at it the right way. For instance, some reasons why they've reintroduced walkers on a large scale could be:
1): Threat of public backlash. Many of the reasons why Nod globally were as successful as they were was because of their transition in design from military to environment, and their actions to cut back expenditures - prime examples of this being Goddard in the Nod prologue, and the GDI intel database which stated that before TW3, they had shut down about 60 or 40% (can't remember which one exactly) of their worldwide bases. Because of this there was a lot of civilian casualty which could have been avoided had the GDI council dared to be more expensive. Safe to say, they can't afford to be cheap any more and still expect the people's support.
2): New GDI leadership has been established. The previous GDI council was destroyed with the Philadelphia, so there would be a whole new set of faces who - either for their own concern on the matter or simply to get a good face with the public - would give more support to the military and show a stronger resistance to financial cutbacks. Granger or Boyle - depending on which GDI ending is canonical - would most likely back these with military reputation and know-how in the former's case, and a sound financial management ability and endeavour in the latter.
3): The success of Nod's Avatar walker. That an army whose doctrine is guerrila warfare, beating your opponent not through head-to-head brutality and hit-and-run strikes, can actually challenge a force as strong as GDI head on and win with just one type of unit - the Avatar - acting as the backbone of the Nod force, and beat GDI at their own game, is quite a feat. It's certainly proof that Walkers - or the Avatar at least - are no pushovers and can indeed be the front line of an army whilst being cost-effective.
