One of the complaints included in the Dec'04 report from <email deleted> was that spell of charm monster doesn't give
any feedback. This makes that spell, and scroll of taming, always give a
message. And the scroll will become discovered if a visible or sensed
monster gets converted from hostile or peaceful to tame, or from hostile
to peaceful.
Scroll of taming/spell of charm monster didn't hit steed when hero
is mounted; now it does. I don't know whether that matters though. You
can saddle a non-tame monster, but you can't ride one even in wizard mode.
However, I'm not sure whether a tame steed you're already riding can
become untame, let alone if it can do so without throwing you. If latter
is possible then re-taming while still mounted has now become feasible.