This is a lot easier said than done, as the Australian referendum on abolishing the monarchy showed.
The difficulty is that it requires a substantial re-write of the Constitutions. You can’t just strike out “Queen” and put in “President”. You always have to answer at least two major questions: how do you select the President? and what powers do you want the President to have? Those points of detail are the difficult ones.
For example, in Canada, the Queen has extensive constitutional powers, both prerogative and written. If you want to keep the focus of the Constitution as a system of responsible government, with the new President as a figurehead, and real power resting with the Prime Minister, you have to have a system of election that does not give the new President the political legitimacy to use those enormous powers. You also likely have to define the principles of responsible government much more carefully in a written constitutional instrument, rather than relying solely on constitutional convention, because now you would have two different politicians squabbling over power, rather than the unelected representative of a hereditary monarch deferring automatically to the elected Prime Minister.
But if you want the President to be popularly elected, or to have real powers, then it’s even more difficult - that would be a wholesale change to the constitutional and political system. Those types of fundamental constitutional changes are typically not easily made. The most difficult is to combine popular election with the system of responsible government - you might end up with a completely hybrid system, more akin to the French or Russian governments.
The difficulty in switching from a constitutional monarchy to a republic is well-illustrated by this comment in the wiki article:
(My emphasis.)
In short, abolishing the monarchy and switching to a republic is easy in theory, hard in practice. So I’m not convinced that as soon as Her Majesty is no longer with us, you’ll have a rash of Commonwealth Realms suddenly going republic.