Companies like Bell, DISH and DirecTV can change over quicker because they aren't dealing with commercial head-ends.
I suspect that the commercial head ends are not that big an issue this time around since many are probably already upgraded to handle existing MPEG4 channels.
Bell also took several years to complete the switch. They delayed for at least two years after the initial announcement and then took another year to complete the receiver swap. I suspect that much of the delay was to reduce cost and liability for replacing incompatible equipment and receivers. There were also technical issues with signal encoding, satellite capacity and satellite reliability at the time. The longer Shaw waits, the more likely it is that incompatible receivers are taken out of service and customers will have replaced equipment on their own.
Another issue is customer attrition. Satellite TV customer numbers are dwindling, especially in urban areas that are served by cable, DSL2 and fibre. If there is any cost to customers for new receivers and equipment, they are just as likely to switch to the competition as stay with Shaw. A badly executed transition could spell the end of Shaw Direct as a viable service.