There was a thread with almost the exact same title and theme posted just two days ago, which earned a lengthy reply by Maple Nectar. If you truly haven't seen it, I recommend giving it a solid read, as it goes into detail as to what goes on with more radical changes like what happened with Nunu or Galio.
The TL;DR is that changes like these are focused on changing a champion's design rather than their balance, and are therefore less likely to be balanced on release. Riot deems this acceptable because the following patches are then a matter of adjusting the numbers as needed, rather than developing new mechanics. They also understand how this can negatively affect the pacing of the game when players aren't ready for more disruptive changes, which is why they're planning to change patch cycles: the new iteration would still feature a patch every two weeks, but each patch would alternate between one that focuses on gameplay rather than balance, namely by adding, changing or removing mechanics on champions, and one that focuses on balance over gameplay, with a pure focus on adjusting champion stats as needed. Therefore, it would become more predictable which patches would be more disruptive, and which patches would truly aim to improve balance.