Using RUP methodolgy, when doing "finding actors and use cases" and "specification of use cases" activities, we are recommended not to think about how we are going to build the system (take design decisions).
But later when doing the design discipline activities sometimes we find some design decisions lead to changes in the scope of the system, normally including secondary actors and new paths in some use case specification.
We are building a system that send documents to signers when a primary actor requests that. During the design phase, when thinking about how to build the system, we found it is a good idea to use some existing signer provider.
Now the system we are building is going to send the document to the signer provider and the signer provider will manage the sign process with the signer.
So responsibility is moved to this new secondary actor. Then we need to specify the interaction between this secondary actor and the system we are building, as specifying what happens when the signer provider fails communicating our system some event etc. Should we update the requirements artifacts and show them to the users and client? Is this situation common?.