Upgrade support for Sellable

A step towards adding DeployToUpgrade.
This makes me uneasy for two reasons.  Most traits use `.Trait<>` after checking for `HasTrait<>` (or more efficiently, use `.TraitOrDefault<>` directly), which means that the only use-case for this is for checks like the one below.  I don't like the idea of introducing a matching `EnabledTraitOrDefault<>` to mirror the existing queries, as this goes against the principle of defining an minimal orthogonal API.