Note: it may be necessary to reinstall Miniconda from scratch if packages break during a Python version upgrade. Consider this before attempting an in-place Python upgrade. There is often a couple month delay between a major Python release and Anaconda defaulting to the new version.
Use the new Python version in a new conda environment by:
conda create -n py3x python=3.x
switch to this environment by
conda activate py3x
Legacy hard-coded GUIs using external libraries have considerable overhead to maintain, and suffer bit rot far faster than the underlying code. At the very least, be sure your code is usable from the command line and/or as a plain importable module without requiring a graphics library. In stripped-down cloud / embedded applications, unused graphical imports cause stability problems and considerable startup lag.