Your Ubuntu base see’s the upgrade path open so it’s offered to you.
However, the Lubuntu team recommends you don’t take it.
On each release notes from Lubuntu 18.10 to the latest Lubuntu 20.04.1 LTS you’ll see a paragraph like the following from 20.04.1’s release notes
Note , due to the extensive changes required for the shift in desktop environments, the Lubuntu team does not support upgrading from 18.04 or below to any greater release. Doing so will result in a broken system. If you are on 18.04 or below and would like to upgrade, please do a fresh install.
How broken it will be, will depend on what packages & changes you’ve made to your prior LXDE system. It may be obvious, or the problems may become more apparent over time (minor glitches becoming more serious).
Yes it’s possible to upgrade (I did it from 18.04 to 18.10 following documentation written by the Lubuntu team during the development cycle), but the problems took time to solve before my machine was back to fully-usable. Experiences like mine meant the upgrade cycle is not supported (ie. you should expect problems, and you better be prepared to deal with it yourself due to unsupported nature).
A clean install does give a cleaner experience, is faster too (install is far quicker than a release-upgrade, even if you are restoring some user files from backups).
If you want to limit the restore time, you can use Manual Partitioning and use existing partitions without format checked. This will cause system directories to be wiped, but user files won’t be touched. It may not be as clean an experience, but is my normal ‘backup’ option if I have problems with release-upgrade. I have limited experience in the LXDE -> LXQt switch (but have used it on a few boxes), however any issues that were left I’d have just fixed myself easily. These issues were minimal (I don’t remember them) when compared to the release-upgrade issues on my primary box. Your experiences may of course differ as I bet anyone reading this will have different packages & configs to my own choices.