It took me 2 tries to realize there are several hoops to jump through, but I got it done at eurodns
Steps basically:
1) Create New Profile (Update DNS profiles > Add New DNS Profile)
2) Edit Details for each domain (> Modify, name servers)
3) Select the new profile and click "Use This DNS Profile" below the profile box AND THEN click the "Use This Profile" button below AND Click "Perform NS Check to Submit Your Modification" (not sure on order of these last 2).
4) After all those clicks you come back to the details of your domain with the "current" and "modified" values. SCROLL TO BOTTOM OF PAGE and click "Validate All Modifications".
That's all!
Simple as 1, 2, 3 (a,b,c), 4 (a,b) !!!