benny59 wrote:1) saving the home dir is enough ? (how shall I rebuild the user after the installation)
Copy each users home directory to some external storage device. The home directory contains a lot of hidden files and folders that include default application settings for some of the default KDE applications, your Firefox profile, and a bunch of others. I usually login as root from the login manager the first time I start the computer up after the install and copy all the contents of the home directory back over to the new user that was created during the install, then logout and log back in with the regular account.
benny59 wrote:2) how can I save any additional application I might have installed (i.e. evolution) ? shall I reinstall everything ?
You will need to reinstall additional applications.
benny59 wrote:3) How can I rollback if something goes wrong ?
Look at an application called remastersys.
benny59 wrote:4) In six months time shall I repeat the same procedure ? it does not seem very practical as LinuxMint is my "production" OS the only installed on her laptop (please consider my wife, not IT person, was educated on linux and I am proud of it).
Keep in mind that you don't have to upgrade every six months. I know several people who only use the LTS releases and are perfectly happy that way. Some other people I know only upgrade once a year.