Is it possible to update from Mint Mate 18.2 to current Cinnamon release without fully backing up all the data files - or will such an update always over-write all details on the hard drive?
One should always make a backup in any upgrade/update scenario. No matter how theoretically safe it may be.
Welcome to ‘It’s Foss’ @Geoff_Phillips. As @Akito pointed out, one should always have a backup of their data and also a backup of their system (OS).
But to answer your question, Yes, you can upgrade to Linux Mint. You would to have to first update your system to 18.3. This can be done by going to the ‘Update Manager’ and then to the tap ‘Edit’ you should see the option to upgrade to “Linux Mint 18.3 Sylvia”. Linux Mint 18.3 is supported until April 2021.
The procedure to upgrade Mint 18.3 to the current Mint 19.3 you will have to do thru terminal commands. See https://itsfoss.com/upgrade-linux-mint-version/
Personally, I would do a clean install of 19.3 from a live USB. Linux 19.3 is supported until April 2023. But if you wait a month, you could install Mint 20.0 and it will be supported until April 2025.
I would go a step farther than Akito–ANY data that is more than temporarily useful goes to an external device. I routinely go through Documents, Downloads, and Pictures and move files to an external drive so that I’m always insured against any kind of disaster, including my own finger-fumbles. Minimizing the damage from my own mistakes maximizes my protection against random accidents.
Welcome to ‘It’s Foss’ @berninghausen. It’s a Very good policy to backup your data to an external device. So many times people complain about something went wrong and their PC files are lost. And it can be caused by many things. Hardware (HDD going bad), software (updates / upgrades) and yes our own finger-checks (fingers-fumbles).
I might go over board with backups, but
System (OS) file - Timeshift on a separate partition both daily and weekly. Clonezilla backups to offline (external) storage. At least 2 copies.
Personal (My) files - again, backup to separate partition, and 2 copies on external devices. For family pics and pics from vacations, 4 copies.
Thank you all for your feedback on this matter.