Why is it not necessary? Seriously, I know characters who were tier 5 and became tier 2 with almost no training.
And this wasn't even something exalted in the work, or actually said "wow, he got so strong with almost nothing!". No, he was simply tier 5, and then became tier 2 with almost no training (normal training that he's done before and never made such a big jump in power), and what did we do? A new key, not an upgrade to its previous version.
I know another who was 7-C and didn't do anything (not even training) and in his next appearance he became 7-A.
Sonic himself evolves a lot with that shadow technique, which went from 4 to 10.
Or Ian another OPM character who with just training and without the arm he previously had, he jumped from 8-A to High 7-C