Transition- Convert Ubuntu 16.04 to Lubuntu 18.10(without internet)

Solution 1:

The most efficient way to transition from Ubuntu 16.04 to Lubuntu 18.04 is to back up everything (documents, pictures, and so forth) you want to keep and do a clean install. This will remove all the Gnome components that were installed for the desktop in 16.04, as well as other packages that might also contribute to bogging down your system, and replace them with the "lightweight" Lubuntu components of the new version -- and do it with a minimum of fuss.

Once you have Lubuntu 18.04 installed, you can easily install other software you need that didn't come in the Lubuntu installer -- you'll have access to all the same repositories you did from Ubuntu 16.04 (except newer versions of almost everything).