@Lylec -- in my opinion, it EVENTUALLY will happen, if the take-over actually happens.
If Shaw has 1 million customers, it will take Rogers a long time to achieve 1 million "migrations".
Note that there is a difference between still being able to use your "@shaw.ca" ID, and being migrated into "Rogers Yahoo Mail" - the system that current Rogers Internet customers are using.
Like the Wizard of Oz, what happens behind the curtains may significantly change, if you are allowed to keep using your "@shaw.ca" ID. Just don't look behind the curtains. 🙂