Sometimes I do an upgrade and I want to get the latest security updates, but I also am not really available to burn through a lot of cpu time, (especially on my x200). I'd love it if thre were a flag so that I could specify "only bother to upgrade the packages where we only upgrade if a substitute is already available. Note that I looked at the source to see if this would be an easy thing to add; I figured that this would be handled in roughly the same place as --keep-going or --fallback, but it looks to me like those are actually just passed over to the build daemon, so I'm not so sure how easy it would be to patch this in while we're still using Nix's C daemon? I'm not sure.