The schema resolution page says:
> if both are enums:
> if the writer's symbol is not present in the reader's enum, then an
error is signalled.
This makes it difficult to use enum's because you can never add a enum value and keep old reader's compatible. Why not use the default option to refer to one of enum values so that when a old reader encounters a enum ordinal it does not recognize, it can default to the optional schema provided one. If the old schema does not provide a default then the older reader can continue to fail as it does today.