Will package managers remove all traces of the old DE? Back in the day, `apt remove kde-desktop` would not reliably reverse the effects of `apt install kde-desktop`.
You can certainly remove packages that were installed as dependencies, even if `apt remove` doesn't do this by default. I think it's `apt autoremove` or `apt purge` (although I haven't used apt in a long time). All of the package managers I've used have a way to do this.
On the other hand, for the average user I don't know why you'd bother. It's not like it's interfering with other stuff you want to do, unless you are extremely tight on hard drive space.
apt doesn't remove the settings in your home directory. So you need to nuke them and reconfigure the entire desktop and switching DEs definitely break stuff due to file type handling and default apps. With Xorg there were other things like styles that got permanently broken unless you hunt for every file that has been changed.
