Does the server know about the modified spell.dbc?
that is done by just adding it to the server .dbc right?
Yes. You need the updated spell.dbc in both your clients patch and the servers dbc folder.
Modding don´t work without patches. So thats not the problem
I meant client-side patches vs server-side patches. In my opinion the general audience (again not on modcraft) prefer to not have to download content and would rather just plug and play with a simple realmlist change.