Technically, the ward numbers don't change until the new council takes office on December 1, 2018. A Councillor would continue to represent his old ward number until then, although they could be the "Councillor for Ward x" and "Candidate for Ward y" at the same time, just to confuse everyone.

Looks like we will have a challenger in Ward 6 this week: https://twitter.com/MissAmberTO/status/993851871310757889