How to deal with breaking changes in nixpkgs?

[xaverdh]

Service definitions do break in nixpkgs from time to time (e.g.. with releases).
nixos-manager needs to somehow deal with this.

[xaverdh]

Actually I did give this some thought, and came up with the following scheme:

The services tab might show not just the options available in current nixpkgs, but a union of those and the options already set in services.nix. When an option is present in preexisting services.nix, but not in current nixpkgs, it would get visually marked somehow (color / a note or such), which tells the user that its not / no longer supported in nixpkgs and building the system will likely fail.
Also there could be a ui element for removing the option from services.nix at the users request.