Extended grapheme cluster stability

Hello,

None of the *_Break properties are stable, as far as I can see in
https://www.unicode.org/policies/stability_policy.html. If I understand
correctly, this means that, at least in theory, it is possible that in
Unicode version X a sequence of characters AB forms an extended grapheme
cluster, i.e. A × B in the notation used in the algorithm description
and in the test data, but then in Unicode version X+1, that changes to A
÷ B.

Am I reading this correctly or is this not possible? Or is it possible
in theory but not in practice? Or maybe it has happened before?

-- 
Martinho