In the February 27 Federal Budget there was a mention that the government will be seeking permission to remove the legal tender status of banknotes issued since 1935 that no longer circulate.
https://globalnews.ca/news/4052577/old-canadian-banknotes-legal-tender-budget-2018/Initially once legislative approval is given only the $1, $2, $25, $500 and $1,000 notes will be effected.
Aside from the two lowest denominations no one in their right mind would spend these bills in a normal transaction and like the more recent paper notes they're being withdrawn regardless so why all the fuss?
https://www.bankofcanada.ca/banknotes/upcoming-changes-to-legal-tender-status-for-older-bank-notes/I understand that the B.O.C. might not want these on their books but doesn't that amount to an indefinite loan? The majority are held by collectors or as souvenirs in addition to those lost or destroyed over time.
This is the most logical place to start as it will force average Canadians to dump their stash of notes at a local bank with the least amount of disruption. Many however will probably interpret this as demonetization.
Sounds like this will eventually extend to all denominations that are not current circulating legal tender so that only the polymer series will be recognized similar to the what the Bank of England has done.
