There was an evolution, actually. First of all, Mitsuhiko getting his crush on Ai was something that didn't automatically happen. And Haibara didn't always tease him or smile at him, and Mitsuhiko jumps more often at the chance to protect Ayumi or Haibara.
I think that's evolution in a relationship.
And Ai and Conan don't have that much in the first place. They may have had character development- but it doesn't show. Nothing has changed between the two of them EXCEPT for MAYBE the fact that Haibara trusts him a little more. And judging by the fact that she sent his parents after him to London, I think that says something to her trust of him.