How to upgrade to LMDE 4 - Revisited

Upgrade instructions vs. real world experience

I've got a dirty little secret. I'm running Linux Mint Debian Edition and have been since end of May 2019 despite my known stance on a particular init system. For various reasons LMDE 3 ended up on this new laptop and I've been very happy with it ever since. It does everything well, Steam gaming, Virtualbox and the more mundane tasks like writing this piece. Don't change a working system - so I stayed on it. But a new version has been released and it was time to upgrade.

The Mint team have provided a good guide on how to upgrade to LMDE 4 which works really well when followed but you still might face some issues along the way. It cannot be repeated often enough: Stick to the upgrade instructions and you'll be fine. Also, read the notes under F at the end before you commence with any of these steps.

First, complete all the steps under C as directed, from C1 applying all package updates, creating a system snapshot with Timeshift, to downgrading and then removing all multimedia packages from the Deb-Multimedia.org repository. That includes packages such as Kodi, VLC and codecs. Lastly, remove all other foreign packages.

Important: Make a backup of your sources (and, less important, imported keys) so as to be able to re-add external repositories at the end - the mint sources tool can fetch missing keys later. 

Proceed to steps listed under D and install the mintupgrade tool. "mintupgrade check" is great for a dry run  and is utterly reliable. Kudos to the Mint team as something like this really helps. You'ĺl most likely notice an issue with  libservlet3.1-java which will need to be removed before the upgrade can continue as it will otherwise stall here. Remove it with  sudo dpkg -r --force-depends  as suggested in the comments. After this the upgrade will work fine and should not require any further intervention.

However, it cannot be stressed enough, the system will become unresponsive for a longer period of time as noted. The Mint team reckons this may be up to 10 minutes, it might have been even longer for me even on an eight core machine with 16 GB Ram. The important thing is to be patient and wait without panicking. It should not be necessary to log out and interrupt the upgrade process or drop to a console. Give it up to 30 minutes.

The other thing is packages. I did not remove foreign packages that were not indicated as necessary to be removed to continue with the upgrade. As I found they were removed during the upgrade process anyway so be aware of that, together with external repositories (the Opera browser, for example).

Thus there should also not be a need to remove any more foreign packages after the upgrade has finished. Packages that were automatically removed were for example Steam, Virtualbox (from the Oracle web site), manually installed Master PDF editor, Seamonkey (the Mozilla build of Seamonkey from Ubuntuzilla) and I believe Google-Chrome.
Also, some native packages that were in LMDE 3 are missing as the upgrade enforces its own recipe for a standard LMDE 4 install. This means we no longer have a text editor, a choice I personally do not understand. Xed is still in the repositories so I reinstalled it from there for the sake of consistency, but any other text editor like mousepad, leafpad, Joe or gvim will of course be fine. Come to think of it, the upgrade may have installed Gedit which I would have removed immediately.

Finally, re-add all your favourite foreign and manually installed packages.

The upgrade was performed about a week after LMDE 4 came out but it took me a while to write it up, which gives the added benefit of having being able to test the upgrade in daily operation. The system has been utterly stable, fast and responsive in the approximately two months since. Everything works as intended, incl. gaming and some remote management of servers. No crashes or unexpected or weird behavior. Great stuff. Thanks, team.