Is it really that great a mystery to you? Is this your first cruise? Ever? The ship has to be turned around, provisioned, sanitized, and perhaps inspected by Canadian authorities.
And what difference would it make for you to know the revised itinerary 24 hours earlier? Why the urgency?
One can hop off the hop on/hop off bus to explore the city and experience the city? Would you prefer a regimented coach tour? Perhaps Silversea imagined that the hop on/hop off bus provides optimal opportunity....
Stuff happens in life. It seems to me that Silversea has done a very decent job addressing this unfortunate situation.