@juju read the F.a.Q.
To all but “I” you may change as it’s described in the F.A.Q.
If you want the international firmware, you may find a guide in the General Section.
But having the “I” firmware on Chinese phone has draw backs, as described in the guide.
My recommendation is, unless you want a specific language, that’s missing in “A”, stick to the " A" firmware.