From the MTO's web site:
Vehicle Description ChangesYou must report a change or data correction for a registered vehicle within six days of the change. These changes include:
- Colour
- Body Type
- Fuel Type (Power)
- Number of Cylinders
- Registered Gross Weight
- Make, Model and/or Year
- Vehicle Identification Number (VIN)
Bring your vehicle permit to any Driver & Vehicle Licence Issuing Office and complete an Application for Registration indicating the changes.
This implies that an engine swap to/from the same number of cylinders, does not need to be reported.
As for those taking the 4-6, 4-8, 6-8 leap, if you plan on registering the engine swap with the MTO the easiest route to take would be to phone your local licensing office to find out what documentation (if any) is required, rather than going on rumour and speculation.