The Residential Tenancies Act, 2006 sets what you can and cannot do about it. A fixed term agreement runs with the property and does not end because the house sold, and a buyer wanting to occupy the unit has to follow the notice requirements in the Act rather than simply asking the tenant to leave. Deposits, prorated rent and utility arrangements all get adjusted between seller and buyer through the lawyers.
A below market rent is the quiet problem. A long standing tenant paying well under current rent depresses the income an investor can underwrite and makes the property unattractive to an owner occupant at the same time, which is the worst combination. Getting a written estimate of market rent before you list tells you how large that gap actually is.
The other half is legality, since only a permitted suite produces income a lender will credit toward qualification, and the City of Saskatoon holds that file. Access for showings is a practical constraint too, because a tenant who will not cooperate limits your market. Joel Dyck works through the occupancy question with sellers before listing, then prices the property for the buyer it will actually attract.