[joncrocks]

And I'm not discussing changing the schema, but the data contained with the data structures.

When transforming the schema, you frequently have associated changes that you apply to transform the data from one form (in the 'before' schema) to another (in the 'after' schema). These transformations are code that can have bugs like any other.

In cases like these you can have data that is in the right format, but isn't correct, and can need a second change (to the data only) to correct it.