Ubuntu – Is there any “official” guidance to upgrading from Lubuntu 18.04 LTS (LXDE) to Lubuntu 18.10 (LXQt)

From the release notes for Lubuntu 18.10:

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.

Appendix D Upgrading from previous releases deals precisely with this issue.

Guidance is provided for those who want a GUI route and for those who prefer to use the command-line. Anyone considering upgrading can consult the link for full details.

Of note is the caution that, at some point during the upgrade, the user will be asked to choose between display managers:

You will be asked to select a display manager, the new default is Simple Desktop Display Manager (SDDM). This will happen in the middle of the install, so don’t walk away until after you have done this.

Another point to note is this:

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.

There's a terminal command to remove what the Lubuntu team regard as unnecessary software:

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

Running

sudo apt autoremove

after the removal of the packages listed above would clean up further.