Originally Posted by SparseFlyer

Just a couple things important to note:

1. There's no such thing as an account with credit with AC
2. Most restrictive rule applies on international tickets.
3. When doing an exchange, if the previously used segment was a partially combined round trip fare (which is nearly all cheaper international fares, RT fares combined on a half RT basis), whatever you decide to change the return must be for a fare that can be partially combined with the original outbound. If it isn't, the original outound must be re-priced at the level you would've paid had you purchased one of the fares available at the time that was a one way fare. Then you can use the residual value (if any) to pay for a new itinerary.
4. Partially used tickets can be exchanged to something else, but all travel must be completed within one year of the original departure date. Not one year of cancellation.

So unless OP decides to cancel his YYZLHR return flight to specifically do a one way between Canada-Europe to a destination that is combinable with the original outbound, and return at least within one year of departure, I would say he is better off considering the cash gone. Even changing to just a later date can be tricky if his original outbound was a 3 month max fare and he returns after 4 months.