Purely hypothetical, but to some degree realistic. None of us know "the day".
There is generally a lag of a few years, but since the journey series does not only feature the queen, the question becomes, would they integrate the new monarch onto the $20 while keeping the rest of the notes the same (which is what I think would happen),
OR
Would the new monarch get "his own" series of notes? Like a sudden end to the Journeys perhaps.
As far as I can tell, the issue of separating from the monarchy is really a non-issue for Canada at this time (or at least on the back burner). Wouldn't the security features carry over to the next note style anyway? All that would be changin realy is the picture and maybe some minor details.
Anyway, the bottom line is that we all gotta go sometime. I think they would introduce one of the Princes on the $20, and then after a few years when the journey runs its course, then there may be some changes in who is on what bill. Maybe prince Harry/William wants to be on the $100?
On that note, do they even have any say in the matter?