Airworthiness Directives are routinely sent to operators of commercial aircraft, and could relate to any component of any airplane. This is not the first nor the thousandth AD received by Air Canada. As explained above, the operator must comply with the AD within the stipulated timeframe/cycles, or else the plane doesn't fly.
I don't know how long it takes to replace an interstage seal, or even if AC is affected by this AD, but this is the sort of thing done overnight when the planes are towed into the hangars for routine maintenance. I imagine the only likely disruption is possible revised scheduling & prioritization of various fins by the maintenance planners.
Originally Posted by WaytoomuchEurope
Scheduled to fly a 777-300 in 3 days.
It's unlikely any of the 777s will reach half the number of cycles within 3 days.