I have decided to remove GNOME and install Unity in my Ubuntu 18.04 and I need some advice on how to do this without affecting my personal files, installed packages and other installed extensions.
Can someone please help me with that?
You could proceed as following. Caveat: do this (as usual) only after you have a full backup of your data. There is always a possibility something goes wrong.
sudo apt purge gnome-shell
This will completely remove packages related to the shell. This will also remove the ubuntu-desktop metapackage, because you indeed are removing a core part of the current Ubuntu desktop. This will also remove the gdm login manager.
sudo apt autoremove
This will remove all packages that were automatically installed but are no longer needed by any remaining package.
ubuntu-unity-desktop) is properly set as your default login display manager by running the command:
sudo dpkg-reconfigure lightdm
(see How to change from GDM to LightDM? for specifics)
sudo apt install --reinstall ubuntu-unity-desktop
Thus, Ubuntu will reinstall the (small) metapackage, and while doing so, check that all required dependencies are (still) in place. May not be needed, but just to be sure.