How to tell Timemachine that the Machine Dir has moved?
I think I have found the answer. The following site was very helpful:

http://pondini.org/TM/18.html

There are important differences between how Time Machine backups to local disks, and tonetworked disks. In the first case the backup goes to a "Backups.backupdb directory, in the second case to a "sparsebundle". 

So my solution will be:


First have Timemachine start a new back up to my now networked
Drobo. 
Interrupt the backup after the sparsebundle has been made.
now move the Drobo temporarily back to my Mac, mount the sparse bundle, and use diskutil to erase it.
Copy the existing back up in to the monted sparsebundle.
Attach the Drobo again to the Time Capsule once this is finished.


However I can't try this out yet. First have to expand the disk space on my Drobo. So the first step for me really is: Get two 4TB disks on Monday...