Does Mastery 4,5,6,7 even mean anything on reworked champions?
Imagine you're playing your favorite main champion Akali one hour after her rework, and you flashed your M7 emote. But what does that emote even means anymore? You're playing a completely new champion, that mastery doesn't mean shit anymore.
To avoid such confusion, Mastery should be reset for specific reworked champions, but the Mastery Points earned from playing matches should be increased. And when reached their recently unlocked M6/7, they only require to finish 1 S-/S game for M6, and 1 S/S+ for M7.