Basic assumption behind a service contract is that the interface would not change once it is published. However, unless there are other implementations of the service contract (so people, speak up in that case) I would suggest that you go ahead and change the current service contract (better manage an interface rather than many). In case people wrote implementations based on the current contract, it would be better to fork that service contract with the appropriate changes into a new one (in which you would have the flexibility to add/remove more things).
Given this opportunity, as soon as I have some free time I intend to post about some changes/thoughts on the acs-service-contract package that I had in mind (OT: btw, if someone is interested to port my RSS package into oracle, pls email me, so that we can commit it to the repository. Currently, it's postgres-specific and the UI has to be rewritten with openacs templates but that's not too much work).
