I wrote a 20+ page postdoc grant last year and was rejected, but decided to revise and re-apply with the same collaborators. I am probably days away from it being ready to re-submit, with slightly better odds this time around. In total it's gone through maybe 4-5 months of work with multiple drafts making the rounds among myself, the PI, and 2 other collaborators. However, I just got a postdoc offer that is too good to turn down (funded position at the perfect balance between learning new skills in a new system while flowing naturally from my PhD work). I know I will accept this new offer, so how do I contact my collaborators on the grant we were writing to turn them down? The timing is awkward as hell. Should I just submit it (since it's nearly ready) and then once the new offer is finalized (signed contracts, etc) tell them? Or, tell them now and propose that we sit on the proposal for 2-3 years to submit around when the first postdoc ends? I don't want to burn any bridges as the grant collaborators are also important names in the field and I'd like to keep future collaboration possibilities open.
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 "diplomatic" response, just an honest one. If you can stay connected with the project, if even as the "idea" person, it should be fine. But you will need to make a commitment that you will do "something" for the project even though you will be busy with the new duties. Collaboration with people from different institutions is normally a good thing.
The most diplomatic approach I see would be to tell your current collaborators that you are taking another position, but that they should feel free to move forward on the grant on their own, either by finding a new postdoc to fund with the proposal, or to morph the grant into something where one of them acts as PI, so their effort in helping you polish/shape the grant isn't lost.
Of course they might be willing to wait a few years for you to get around to working on the project with them again, but I doubt it. First, science moves on and someone could beat you to the punch or the proposal may simply be less relevant in 2-3 years. Second, who knows whether you will come back to them in 2-3 years?
If the research you're proposing is still a match with your new lab, and you think a productive collaboration could be formed between the two locations with you acting as the bridge, that's another great option. However, it would require substantial commitment from both labs, and you would likely need to work a substantial amount of overtime to get everything off the ground in addition to making adequate progress on the project your new boss is hiring you to undertake. Whether that's worth it to you depends on your confidence in the project, your collaborators, and other situation-specific variables.
If you decide to go that route, I would discuss it first with your current collaborators. They are the ones with the most invested in the project at this point, and they're the ones who should get to decide where, when, and how to move forward with or without you. Since the proposal is privilged information, only take the proposal to your new boss if your current collaborators have all signed off.