As I recall, the identity of the Founders was going to be revealed later, but Piller (I think) pushed the writers to go with it right away.
I'm torn on that idea. On the one hand, it was weird to go from knowing almost nothing about the Dominion to knowing their biggest secret over the course of only three episodes, stretching out the mystery of the Founders across season 3 would have made more sense. In Babylon 5 the Shadow War was in full swing by the time we learned who the Shadows really were and what they were up to.
On the other hand, the Changelings are the best thing about the Dominion. The Jem'Hadar and the Vorta are rather generic as alien concepts and their most interesting attributes are their devotion to the Founders and "the order of things". The Changelings not only have an interesting backstory and truly alien society, their abilities also make them a very clear and present threat. But perhaps not having the Changelings to fall back on for a season would have allowed the writers to develop other races of the Dominion and given a sense of them as an anti-Federation as originally envisioned.