The steps that I imagine I need to do would be:
1. Run migration with rs.exe.
2. Update credential information on the destination server in the data sources.
3. Verify all reports on destination are rendering correctly.
4. Disable subscriptions on the old SSRS server.
5. Once new server is verified to be working correctly, the take offline old SSRS server.
Am I missing something? Is there something else I should be considering or checking on?
Advice on this would be much appreciated.