Q: Removing a "backports" PPA Hoping to fix a few issues with KDE apps on Kubuntu 16.04, I added a backports PPA (kubuntu-ppa-ubuntu-backports-xenial), and updated manually a couple of apps (apt-get install  --only-upgrade`). 


*

*made very extensive changes to the system (many packages added/removed)

*created more problems than it solved (my fault?)


Furthermore it is about time I apply a bunch of regular upgrades, and I'm not confident enough to allow a general upgrade from the PPA.
So 


*

*how can I remove the PPA with minimal damage? (What happens to the already installed software?)

*Is there any chance that the problems come from either not upgrading my system before adding the PPA (no upgrades in the last two months) or because I used the wrong option to upgrade?


A: For a safe alternative to manually removing PPA's, you can install a tool called ppa-purge
You can install this by running:
sudo apt install ppa-purge

And then to remove the ppa, you can simply run:
ppa-purge ppa:ppa_name_here

What does this tool actually do?
Instead of doing something like 
sudo add-apt-repository --remove ppa:ppa_name_here

ppa_purge is a safe alternative because it gracefully downgrades whatever was installed from the mentioned...
And to answer your question, assuming that the "already installed software" was installed through the ppa... It will be most likely be gracefully removed...
And as to your second question, one may never know what errors may or may not arise from these specific situations...
