As I alluded to on AskUbu, I would expect a simple
sudo apt remove ubuntu-desktop
To do what you want. I haven’t done that recently on a 18.04 Ubuntu base install with another desktop added, so without testing I can’t be sure if any problems are to be expected. I wouldn’t expect any (results may differ depending on changes you’ve made however).
As I also stated on askubu, I’d type the command and scan the output of packages to be removed looking for any potential problems (not expecting any, but better safe than sorry), then proceed. If I noted any, I’d fix them by a sudo apt install either before the ubuntu-desktop removal (which will cause package(s) to be marked as manually installed; thus they’ll remain), OR after it (same end effect, after the remove it may require the package to be re-downloaded being the difference; tiny bandwidth hit)
For years all my installs were Ubuntu desktop, with my wanted desktop added later as my ISP allowed bandwidth quota free download of Ubuntu ISOs only, updated packages were also quota free so adding lubuntu-desktop (etc) afterwards used none of my monthly bandwidth quota. They no longer offer this so I no longer do it this way.
If you’re worried about the effect, I’d install Ubuntu Desktop on a VM, apt install lubuntu-desktop, reboot the VM, then do the sudo apt remove lubuntu-desktop, restart the VM and look for ill effects; I wouldn’t expect any, but currently I’m unwilling to do that test.
https://packages.ubuntu.com/bionic/ubuntu-desktop
https://packages.ubuntu.com/bionic/lubuntu-desktop