1. You would be due a refund if there was any difference, but for YYZ-LGA there isn't. You are entitled to be moved to a flight that still has Business Class if there is one with availability that is suitable for you (including switching to a JFK service if you want - onward travel costs at your own expense)
2. No you will get the Y class TPs and Avios if the change is made in advance like this. You can't have the higher TPs and Avios and also have a refund of the difference, just unfortunately in this case the refund is $0 Original routing credit is for involuntary changes made mid-travel.