Today we, @thet and myself, created plone-codemod to auto-update old code-bases (like 5.2 - yes we have those still) to Plone 6.1/ 6.2.
It covers Python, ZCML, GenericSetup, Pagetemplates/ Bootstrap, PEP420 namespace Packages and Packaging (setup.py -> pyproject.toml).
Still early, but worked well on our codebase, is expandable and a huge help. Some manual work is left, though.
That's very nice! I will give it a go 
One question about it though: as time passes more updates will be needed, how is that handled?
An entry on the README about how to contribute new method updates, ZCML replacements, etc. would be nice 
At the moment there`s a migration_config.yaml file with all conversions shipped with the wheel. So, this can be extended (PRs welcome). I am pretty sure we did not catch all conversions.
--config is the parameter for custom YAML.
Further development Idea: We could add an command line parameter to point to an "additional rule" file.
EDIT: I added a real documentation, here the Configuration Format - plone-codemod 1.0 documentation
A nice helper 
When I run it (installed in a pyenv virtualenv via pip) I get Could not find plone_codemod.import_migrator in any configured modules though.
$ plone-codemod ../collective.behavior.banner/src
Plone 5.2 → 6.x Migration Tool
Source: /Users/pbauer/workspace/collective.behavior.banner/src
Config: /Users/pbauer/.pyenv/versions/3.13.12/envs/codemod/lib/python3.13/site-packages/plone_codemod/migration_config.yaml
=== Phase 1: Python import migration (libcst) ===
Initializing libcst in ../collective.behavior.banner/src
Successfully wrote default config file to /Users/pbauer/workspace/collective.behavior.banner/src/.libcst.codemod.yaml
Loaded 129 import mappings from /Users/pbauer/.pyenv/versions/3.13.12/envs/codemod/lib/python3.13/site-packages/plone_codemod/migration_config.yaml
Running PloneImportMigrator on ../collective.behavior.banner/src ...
stderr: Could not find plone_codemod.import_migrator in any configured modules
[...]
The other steps work fine.
Same when running via uvx plone-codemod src
This looks like the libcst configuration is not created ... and it isnt, I had it already local from my manual runs. Bug. Stay tuned.
fixed 1.0.0a5 released
Works now. Thanks.