May I ask, how were you able to perform a dist-upgrade despite the puny 4 GB eMMC?
I didn't do anything special. This was mostly the stock image as we got it from you (I think I installed the command line editor "joe" beforehand).
I just checked /var/log/apt/history.log, and it did run out of disk space while it was trying to unpack or configure libperl4-corelibs-perl, so after an initial apt-get dist-upgrade, I had to do apt-get -f install libperl4-corelibs-perl (+ followed by an apt-mark auto libperl4-corelibs-perl to make sure that's not marked as manually installed) to fix that issue, then I did an apt-get clean again, and then a second apt-get dist-upgrade worked. (Obviously it will vary at which specific package this fails, depending on how much space is actually free.)
(Note that in the initial upgrade I only replaced the source.list for Raspbian, not for the Kunbus packages, those came later.)
In its current state (upgraded Raspbian + upgraded Kunbus, but not upgraded kernel), there's 605 MiB free space on /, with ~ 150 MiB used space in /home and 180 MiB in /usr/local. Those additional 330 MiB were not used at the time of the upgrade though.
I'd expect /var/cache/apt/archives to overflow,
No, that wasn't a problem. the entire upgrade did fit in there. (I mean the *.deb files.) I do remember doing apt-get clean before and after the dist-upgrade - and in between both invocations of dist-upgrade, but that's about it.
did you mount that on a USB stick?
No, no external storage at all, just the internal eMMC.
Did you run into any non-trivial issues apart from the missing factory-reset dotfile?
- During upgrade there were quite a few conffile upgrade questions. Most were related to changes made by Kunbus to the Wheezy default configuration. See below for a list of files that were affected. In all cases I had to manually merge the configuration files because some settings of Kunbus I wanted to keep, others I didn't. As neither package being upgraded used ucf, there was no automatic three- way-merge, so I had to do the merge manually.
- I never attempted to update the kernel (yet), so I don't have any experience there. (I figured that 4.1 was new enough to work for now, as Debian Jessie comes with 3.16 anyway.)
- I got a couple of debconf questions during the upgrade, apart from the conffiles. I don't remember them, unfortunately.
- After installing the Jessie pictory, that worked, but the config.rsc file it generates causes the piControl.ko kernel module that comes with the Wheezy image to crash. (Null pointer deref.) If one doesn't access the process image after that crash, one can still reboot the system cleanly - but trying to access /dev/piControl or unloading the driver will effectively freeze the system. (One can still login, but there will now be unkillable processes around, that even kill -9 won't be able to get rid of.) If one modifies the JSON files manually to not contain the "GUID" : " ...", "extend" :  and "Connections" :  settings in the main Pi module area, then the modified file does work, and at least a DIO module works fine. (That's the only one we currently have.) I expect this problem to go away when I upgrade the kernel and the piControl.ko driver.
- I did disable the DHCP client (via update-rc.d disable dhpcd if I remember correctly) and I did give the Pi a static IP (in /etc/network/interfaces) before I did the upgrade. This was unrelated to the upgrade itself, and the default DHCP/LLMNR is good; in our case a static IP for that specific box was just easier. Thus I have no idea how that functionality would behave on updates.
GUI does work, even with Jessie, but apart from logging in I haven't really tried that (and disabled it). I haven't yet tried the logi.RTS at all (neither the original Wheezy image nor Jessie).
Files where I got dpkg conffile questions during upgrade from Raspbian packages:
Files where I got dpkg conffile questions during upgrade from Kunbus packages:
(Obviously if other people modify more than we did, then they'll get more conffile questions - but these questions were asked even when starting just from the stock Kunbus Wheezy image as it was preinstalled on the .)
That all said: once the official Jessie image is available, we'll likely replace the current installation with a new image - and after we've developed a workflow for creating custom images, using those.