When I switched over to the 6GB plan the rep on the phone told me I had to wait until 12:00 that night and then after that I was good to go. I had a look at my bill and he was right!
I switched from Rogers Vision to 6GB plan. They cancelled my Vision plan immediately upon my call, but my 6gb plan did not come into effect until 12am. I used data between this time and got charged like 30bucks for 1mb of data usage before 12am. I had to call Rogers to get this reversed.