Does security matter to you?
Lubuntu 18.04 LTS is no longer supported; refer
where you'll note only Ubuntu Server, Ubuntu Desktop & Ubuntu Cloud come with 5 years of support; flavors had shorter lives.
You can add
lubuntu-desktop to your existing system, which has the least changes (just adding extra now EOL software to your existing system) without re-install; you can use
ubuntu-support-status to see security status, but do note Lubuntu/LXDE software will be adding more unsupported (no guarantee of support) software to an 18.04 system.
Also note Lubuntu 18.04 was the last LXDE release; so a re-install is required to upgrade to a later system, you didn't specify what release you're asking about that has Lubuntu, you do mention 18.04.6 where no 18.04.6 media exists for any flavors of Ubuntu, as they'd reached already reached EOL (see https://fridge.ubuntu.com/2021/09/17/ubuntu-18-04-6-lts-released/ where you'll clearly see no flavors of Ubuntu are mentioned).
Also FYI: You can re-install, including other desktops without losing files & having your manually installed packages/apps auto-re-install (if available in Ubuntu repositories for the release). You decide at install by the options you use.
FYI: I've QA-tested LXDE on all releases up to and included 22.04, however it relies on many deprecated libraries that are no longer supported upstream, so it's a decision as to whether or not you still want to use it (security wise etc); but yeah it still does work. Modern Lubuntu uses LXQt however, as LXDE developers joined with Razor-Qt developers and creating the replacement LXQt desktop as they blogged about when porting LXDE to GTK3 was deemed to heavy and against the L=Light aim of the project.
Lubuntu 18.04 LTS questions on this site are still on-topic, the unsupported nature I'm referring to is what you'll see with
ubuntu-support-status because only packages from 'main' & 'restricted' come with 5 years of guaranteed support. Lubuntu (and flavor packages are from 'universe' and no longer supported by the Lubuntu team with all guarantees of fixes ending at 3 years from initial release). It's still possible for any MOTU (master of the universe) or a Core-Dev to make security changes until 2023-April, but teams no longer support the packages. About 70% (from memory) of a Lubuntu 18.04 LTS install is from 'main' thus still gets security fixes (I still have a rarely used Lubuntu 18.04 LTS system installed; though I now consider it Ubuntu 18.04 LTS with LXDE & XFCE as I have both Lubuntu & Xubuntu desktops installed)