My OctoPi is running fine, has a a bunch of plugins, some history (STL files, etc.), some stats, customizations in the settings... but it's on jessie which debian's old stable. I would like to have it on stretch (current stable).
What did you already try to solve it?
Look for documentation about upgrading.
Do I have to burn the stretch image and start from scratch ? If so is there a way to export / import some of the stuff I would like to keep ? Can I just do a dist-upgrade like on any other debian ?
Additional information about your setup (OctoPrint version, OctoPi version, printer, firmware, octoprint.log, serial.log or output on terminal tab, ...)
Possibly. But that would only update the Raspbian/Debian stuff, not the changes in customization that happened to OctoPi since your 0.13 version. Which are quite substantial, we are at 0.15.1 now.
I tested the dist-upgrde path a while ago, and s few others did as well. It seems to work without issues, however your milage may vary. If you want to try it, first make a backup of your SD card and/or settings, then find a post by me (I'd link, but on mobile ATM). Due to how much does change though, I would just suggest going then reimage anyway.