They can do all the 333s during one winter schedule when demand is lower. Aircraft are always going through periodic maintenance. If the 333s are due for an H check in winter 08-09, there will be at least one out of the fleet at any given time anyway.

As for the 767s, the combined 767-2 and 767-3 will shrink by four aircraft from the current level by the end of 08 while the large widebody segment, currently 14 (7 777s and 7 A343s) will increase to 17. So in terms of aircraft, the total is a wash, but a number of 767-300s will be released by substituting with a 777. The key thing is that the 777 acquisition will increase the size of the widebody fleet by over 1200 seats from where it stood during this past summer peak, and in situations where AC currently flies or recently flew double daily service with 767-300s (YYZ-LHR, YUL-LHR, YVR-PEK, YUL-FRA, YVR-HNL-SYD...), those 767-300s can be replaced by a single 777 and then redeployed elsewhere. That's how the 1200 seats can turn into 6-7 extra 767-300s. It also appears that adding capacity to the A321s - now up to 172 seats - is meant in part to compensate for reduced widebody flying domestically, which may also help free up some international 767 flying.

I'd also note that if the economy is slowing for 2008 and possibly 2009, AC would not want to start too many new routes. It's best to have 2-3, not five or six since new routes can take some time to turn a profit.