Hello Dries,
btw forget about the addons folder since new snapshot will download latest bindings. However, as i see you want to update stable version of openhab2 to development versions nightly snapshot. Please bear in mind that the nightly snapshots are development builds and might cause you some other problems, think of it as beta versions. Distro versions are the latest stable builds.
You can see the same question in How to upgrade a OH2 distro? and Whats your way to update OH2 with another nightly build?. and as you can see life with openhab on Synology is always harder to use since we do not have apt-get on Synology. Copying /conf always works bu copying /userdata not works so smoothly. What i reccommend is that you create everything on config files like rules, items, transformations and etc. except for things and bindings. What i did is to create some of the things in text file and some on Paper UI; like ntp, weather, printer, hue on text files but Zwave modules on Paper UI since Zwave node management is flawed on text file configurations on openHAB2. So if i update, i copy /conf folder, and recreate all bindings and zwave items in Paper UI and don’t bother with /userdata folder, since you have to delete some files from /userdata folder for openhab to recreate with the new snapshot.
So that means nearly delete and reinstall openhab2! I would personally reccommend that you go with distro editions since in Synology if you do not use the package version (https://github.com/openhab/openhab-syno-spk) openhab2 requires manual installation which i didnot dared to use.
In summary, you have to struggle a bit to do that on Synology. If you are brave enough please let us know how did it go 
And also i realized 2.1.0 004 is a snapshot too 
I still prefer to go with the package scripts and wait for the next one until i fully understand the versioning and updating mechanisms…
I was also wondering since the installer.sh script in 2.1.0.004.spk shows https://openhab.ci.cloudbees.com/job/openHAB-Distribution/lastSuccessfulBuild/artifact/distributions/openhab/target as the download address. So uninstalling and reinstalling the .spk might yield a newer snapshot of openhab?