Steps to Reproduce:

1. Log in to the 'New York' instance as an admin
   2. Open 'Agent Workspace Home' from the filter navigator
   3. In the 'Lists', open 'Incidents' 
   4. Details form will be opened for this incident. In the Agent Assistant in the form, we can see the search resources which are associated with ('Similar Incident's, 'Similar Open Incidents'....)
   5. Open Search Resources from filter navigator (Contextual Search -> Additional Resources)
   6. Open the record from the list which name is 'Similar Incidents'
   7. In the Search Resource form, you can not edit and change the Solution Definition for this search. The searches are associated with OOB Solution Definitions. You would like to associate the searches with your own customized solution definition

Reason:

The reason is admin can modify the existing solution definition with less changes which are executable but not point to another one, it is a known limitation that due to Contextual Search and Agent Assist required behavior only ServiceNow can modify search sources or search actions, there are no plans to change such configuration.