Agile means different things to different people, but to me "the big rewrite" sounds pretty un-agile. You end up making a lot of architecture decisions all at once and go a long time between working releases.
If your business needs have completely changed then it might be worth it, but at that point it's no longer "the big rewrite" but instead just writing a new application. (that isn't required to fulfill the requirements of the old app)
If your business needs have completely changed then it might be worth it, but at that point it's no longer "the big rewrite" but instead just writing a new application. (that isn't required to fulfill the requirements of the old app)