I actually tried it. The simple answer is that you can't.
You should really consider Roman's comment
Why a clean install is not an option if you backup your files and remove apps anyway? Especially if you say that you want a vanilla Debian install.
A good reason to not even try to "convert" Lubuntu to Debian is that any package that only Ubuntu has
(or uses the distribution's version number instead of it's own) will never be upgraded. This is a potential security risk and you may not even be aware what packages are no longer getting updates because Debian either don't have them (with the exact name, or thinks they are very new).
The long answer is that it appears to kind of work with some manual labor to just add Debian's repositories, update everything, remove Ubuntu's repositories and update everything again. A few things fail:
plymouth had to be removed and
linux-headers-686-pae had to be manually installed. Having to manually install the correct kernel suggests that there may be other things missing.
A comment on 32-bit support in Debian
Debian uses the name "i386" for the architecture of packages, the name hasn't been updated but the minimum required CPU has. Keep an eye on ArchitectureSpecificsMemo if you have a really old computer, I wouldn't trust
apt to test if your computer will support the newer packages when upgrading.
Not all packages obey this. One example would be
pypy which assumes SSE2 is supported and it actually uses
/usr/bin/pypy in the post-install and the pre-uninstall script causing it to be a PITA to remove if an installation was ever attempted.
But I think you're more likely to just have too little RAM to do anything graphical.
apt-get dist-upgradeis only removing 15 packages and they didn't even look that important. I do not recommend doing this, it will probably end horribly. This is also a freshly installed and updated Lubuntu 18.04.
/etc/apt/sources.listand concatenated it with Lubuntu's. And the files in
/etc/apt/trusted.gpg.dfrom Debian were added. Then just a
apt-get update; apt-get dist-upgrade. Later I will remove Ubuntu's repositories and compare the system to the host system. I'll write it as an answer when I get the results, but I still won't recommend it.