Two weeks after we moved to our new appartement we got DSL back. I have to admit I was sceptical at first, because ususally I tend to not trust phone and internet providers too far when it comes to make changes to an existing setup. However apparently everything worked well. The ISDN line was switched over on the day we moved, keeping the same number without a glitch. The only thing that got lost on the way was the outgoing caller id. However after two calls to the hotline (hey, this is still well within acceptable limits, isn't it?) they assured me it was to be reactivated by tomorrow.
The DSL provider promised to get the line back up two weeks after the ISDN line had been established and they kept their promise. Today I plugged the Fritz!Box in and everything worked immediately. It is a real relief. I did not remember how slow ISDN was, even with two channels bundled to 128kbit/s. My girlfriend is relieved, too, not having to use my computer all the time, because she did not like to install the ISDN drivers.
From what I see the line should be good enough for 6MBit/s downstream, 3 at least. I will try and upgrade from the current 1MBit/s tomorrow. After all it's just 3€/month and includes a VOIP flat besides the internet flat. Let's hope this works smoothly as well :)