Pulumi will always create a replacement before it deletes the old resource. So if you are directly managing resources like Instances or Functions or Containers, you will get this orderly transition. However, I'm not off the top of my head sure what guarantees Spot Fleet makes around availability of instances at the time the fleet creation "succeeds". If it has not been designed to support this, it may require some extra work to handle this via Pulumi (or really via any system that coordinates Spot Fleets).