This is documented for the benefit of others. Hopefully Google indexes this.
If during upgrade, apt fails and complains leaving you in inconsistent state (this happened for me with texlive cyclical dependencies), try looking at the dpkg message.
I got an error in dpkg where it would refuse to install a package upgrade since it would overwrite a file included in an older version of another package. This was fixed by using dpkg manually and forcing it. Then apt-get -f install was run to continue fixing.
After multiple rounds of apt-get upgrade, apt-get dist-upgrade until no packages were left, there were still many parts of the system that said 17.3 Rosa.
Additionally, many gnome apps were installed giving a very bad experience. Furthermore, things like the system .bashrc (e.g. colored prompt) did not work.
The fix was to rerun mintupgrade upgrade at this point, since apparently it removes gnome apps, installs X-apps and then fixes a bunch of configs (none of these are part of a "package" and thus can only be done by the tool)!
After everything was installed, an issue I ran into was that the Cinnamon Panel became unresponsive. However after some trial and error I noticed that it was just not
getting any mouse input signals (e.g. switching windows updates the panel and pressing winkey/super brings up the menu). Turns out this is caused by ibus!
Here is the issue on GitHub. The fix is also documented, which is to run ibus-setup and set 'Show property panel' to 'Do not show'.
This is bad but, hey, at least you have a workable system now :) This needs some looking into though since the same setup in Fedora 24 does not trigger any errors with ibus.
The video/audio thumbnails are broken as-is. The fix is the following:
If you have ffmpegthumbnailer, please edit the /usr/share/thumbnailers/ffmpegthumbnailer.thumbnailer file and replace it with the following.
[Thumbnailer Entry]
TryExec=ffmpegthumbnailer
Exec=ffmpegthumbnailer -i %u -o %o -s %s -f
MimeType=video/jpeg;video/mp4;video/mpeg;video/quicktime;video/x-ms-asf;video/x-ms-wm;video/x-ms-wmv;video/x-msvideo;video/x-flv;video/x-matroska;video/webm;
The single change is that the parameters are not -i %i -o %o -s %s -f but rather -i %u -o %o -s %s -f! Just change the default %i to %u to take the input file name.
If you don't have this file but are sure you can run ffmpegthumbnailer from the cli then create this file.
Since you're now using xapps (hopefully) you don't have totem so it's xplayer-thumbnailer. Open the corresponding .thumbnailer file and change
TryExec=/usr/bin/xplayer-video-thumbnailer
Exec=/usr/bin/xplayer-video-thumbnailer -s %s %u %o
to
TryExec=xplayer-video-thumbnailer
Exec=xplayer-video-thumbnailer -s %s %u %o
The absolute paths DO NOT WORK with the probing (even though the file is there) causing the thumbnailer to not be run. Make sure xplayer-video-thumbnailer can be run in the terminal first!
This thumbnailer (despite being named xplayer-video-thumbnailer) actually handles both video thumbnails and audio cover art etc. Basically anything that the xplayer can get a thumbnail out of.
With these two patches the video and audio thumbnail generation feature should be fixed for Cinnamon 3.