Q always opens submenu. If there's no submenu, it opens the pickbox or enters the edit mode (those two are actually the same thing). It did that since 2.3, nothing changed.
SET does the most common thing - ON/OFF for booleans, pickbox/edit for 3+ choices. In 2.3 it did pretty much the same thing (edit mode wasn't used for just 3 choices though, it was used for ISO, kelvin, bitrate, audio and most other things with many choices).
So, the biggest change now is that SET uses pickbox/edit even if there are just 3 choices.
In submenus, LEFT/RIGHT were always incrementing/decrementing. This was not changed.