Most of these "we switched from X to Y" stories seem to boil down to Brooks' "plan to throw one away" dictum. In year zero, the technical team of a startup is never going to really understand the requirements and constraints they'll be facing in year three (or even year one). The best you can hope for is to be flexible enough to tear out and replace pieces as you go, and to not have an ego about your original design.

It seems to rarely be the case that it would have been a better idea to build with Y in the first place. In any case, even if you had a crystal ball and could predict your year-four, millions-of-users design in year zero, it'd probably be a mistake to try to build it from the start. The needs of a product (and team) in its infancy are a lot different from those of a mature one.