How do I diplomatically tell my collaborators that I've accepted a different postdoc position?
I certainly wouldn't recommend sitting on the project for a number of years. Science moves on and so do the participants.
But, even if you have to take a different, lesser, role, you can probably move forward with the project.
I don't know if  you need a &quot;diplomatic&quot; response, just an honest one. If you can stay connected with the project, if even as the &quot;idea&quot; person, it should be fine. But you will need to make a commitment that you will do &quot;something&quot; for the project even though you will be busy with the new duties. Collaboration with people from different institutions is normally a good thing.