Recently, we briefly discussed enum schema evolution in the regular ROOT I/O meeting (FYI: @pcanal@jblomer).
My guess here (although @pcanal may prove me wrong), is that the class version number should be incremented if there is at least one data member whose type is enum InitializationType_t.
You don’t need to increment the class version number if you add a constant to the enum.
Enums are serialized as their underlying integer types, so wrt. to the class layout there are no changes. If, however, you need at some point to swap constants or change their value, you’d need to add an I/O customization rule for manual schema evolution.