If I cancel a reward booking, are the points redeposited immediately?

Also, what if I’m booked on two legs, with the first leg on another carrier, eg FCO-FRA on LH, and then FRA-YYZ on AC, and I want to change to FRA-YYZ on LH when that opens up, but at the time I want to make the change FCO-FRA is now I0, X0? Can I modify the booking or do I have to cancel and risk losing FCO-FRA?

And just so that I understand this correctly, as an SE is there no advantage to a business flexible reward booking?