Consider this use case.
Say you have a space with subspaces and documents inside.
You recall that you forgot to make a branch (of spaces) versionnable. After all, this is what you want to use your cms for. So you add the versionnable aspect to the higher space of your branch and check the 'apply to subspaces' (or something) checkbox.
Inspecting the properties of an existing document, you see it is not versionnable.
It would be great if a checkbox existed so that it could apply the aspect/rule to all existing documents.
My 2cts
Jerome