Alberta and Sask will continue to export heavy oil to the US midwest and gulf coast, where most refineries are set up to refine heavy oil.

If it becomes economical for Ontario or Quebec refineries to upgrade to refine more heavy oil, there's nothing stopping them.  They have pipelines they can use to source heavy oil from Alberta.  

But as it stands, Alberta doesn't even sell enough light oil to fill Ontario and eastern refineries, so they'll continue to need to import it.