I believe AC has already announced its plans to continue renewing its fleet on an ongoing basis with 319s and 330s/340s. And to intergrate many of these new aircraft along with any CP fleet downsizing. AC already has a fairly young fleet, with most of the oldest 747-200s being withdrawn and replaced by Airbus 330s/340s, plus 747-400s.
320s/319s have replaced the DC9s on their original routes. And granted the DC9s are old, but they have been completely refurbished with hush kits, etc. and except for cockpit controls, equal to most MD90s in service. (Many of these are actually held in reserve for unanticipated ops, but with the rapid transborder route growth have seen more service than anticipated.)
AC will continue to fly DC9s because they've been fully depreciated and thus operate at a cost advantage even if they are not as fuel efficient as newer aircraft. I would certainly take a DC9 any day over a Canadair CJ on routes across the prairies. So what's the issue?
As for Cdn's fleet, AC has already said it will replace 737s on long haul domestic (i.e. YEG-YYZ) and put F28s on short-haul routes, using the 737s on its new discount carrier in the fall/winter. (And I certainly prefer F28s to the afore mentioned CJs.) All DC10s have now been retired (and as I noted elsewhere, two are sitting on the tarmac east of Edmonton's new terminal building) and Cdn has added more long range 767-300s to replace them. Any other changes in Cdn's fleet will await the settlement of debt restructuring, and this will certainly include terms of the leases held on all its aircraft.