when will python 3.11 or later become the default? We have some SW that's designed to work with python 3.9 and we are worried that SW will break when 3.11 goes live.

How will the 3.11 roll-out process work?

We will make python 3.12 available in AIX Toolbox soon. Probably in 2Q 2025 , we will make 3.12 as the default python3. So "dnf update python3" will make python3.12 as the default python3 and python3.9 and their modules will be still present in the system. Softwares/scripts which invoke "python3.9" instead of python3 will not get affected, but the ones which depends directly on python3 needs checking/testing.
The roll out process is similar to what we have done when switching the default python3 from 3.7 to 3.9.