(wip) BUG: replacing one column's values was changing other columns' dtypes
…s' dtypes

- [x] closes #30512 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


I think the flattening can be done later in the code, will update this later

EDIT
----
update done Actually, I think we can still do better - will update it later this week

EDIT
----
done