I could be wrong here, but I was told it's not normally an issue (although far from easy) to replace like for like i.e. if you already have Pro Nav or Discover Navigation and it breaks, a new system can be fitted and Germany will allow the new serial number of the unit to be registered to the chassis, but if you don't have Nav (or are tying to uplift the spec - say by going to Pro nav from standard), then it won't be allowed.
At least that's what my dealer told me a while ago.