I recently upgraded from Lubuntu 18.04 to 18.10 then to 19.04 using the inbuilt software updater. But I think with the upgrade some of the apps from 18.04 were retained. For example, there are two terminals in the image below:
The switch from LXDE (18.04) to LXQt (18.10) was a huge one, and the Lubuntu team recommended a clean install rather than upgrade for a number of reasons
The most major and notable problem is that upgrading Lubuntu from 18.04 to 18.10 causes a fair amount of issues. Therefore, we are not officially supporting this upgrade path at this time, however we have prepared a page in the Lubuntu Manual which can help address the problems that arise after the upgrade.
The Lubuntu team however wrote notes, for their official manual (https://manual.lubuntu.me/D/upgrading.html)
Please note: The manual now reflects 19.04 which is the latest, but the detail you specifically asked about had this written
After the installation, reboot into the new 18.10 system. You be greeted by a new login screen that is the SDDM display manager and a notification that this is now your first time running LXQt Power Management.
Many of the old applications will not be removed. Newer and often times more featureful, Qt-based applications with the same functionality are installed during the upgrade. Having many unnecessary applications could bloat your system and isn't the true Lubuntu experience, so uninstalling them may be a good idea. The following command will remove unnecessary applications:
sudo apt purge leafpad file-roller galculator gpicview xpad xfburn simple-scan mtpaint pidgin sylpheed transmission-gtk abiword evince gnumeric audacious gnome-mpv guvcview pcmanfm gdebi lxterminal hardinfo lightdm lxpanel lxsession obconf gnome-software gnome-disk-utility system-config-printer-gnome lxhotkey-gtk synaptic update-manager lxpolkit lxtask lxshortcut blueman usb-creator-gtk evince-common
To remove uneeded dependencies enter the following command:
sudo apt autoremove
I'm copying this from the Lubuntu team's phab infrastructure, sorry I don't know how to navigate it as a normal user. I know the manual will show release specific pages in the future, but I don't know if it's already been implemented and or to operate it sorry. It could be this wasn't published officially because of the decision to not-support 18.04 to 18.10 upgrades