It is generally highly desirable that new versions of software should be "Backwards compatible", that is still work on old data artefacts such as XML documents created under a previous version. NeTEx scheme evolution strives to support compatibility with previous versions.

Rules for comaptibility

In the context of XML schema the following considerations apply to compatibility.

The following changes break strict compatibility of XML documents, that is to say, may cause a document that previously validated to fail validation.

• Changing the names of existing elements or attributes.
• Changing the values of existing enumerations (e.g. to correct Typos).
• Changing the default value for an attribute.
• Deleting an existing tag.
• Deleting an existing attribute.
• Changing the order of existing elements.
• Reducing the cardinality of an existing element.
• Making an optional element mandatory.
• Making the definitions of a data type stricter (for example that a name must begin with "'X').

The following does not break strict compatibility, but changes the semantics of the interpretation..

• Adding a default value for an attribute.

The following changes do not break backwards compatibility of XML documents (but may require am updated binding)

• Adding new tags or elements.
• Adding new enumeration values to an existing element.
• Changing comments on elements.
• Adding an XML Group to organise elements in the existing sequence.
• Increasing the cardinality of an existing element.
• Making a mandatory element optional (unless this introduces ambiguity with succeeding elements)
• Replacing a choice with an element whose substitution group covers all the choice elements.

Deprecated changes

Certain changes that break strict compatibility, for example, correcting the spelling of an enumeration or dropping an element or attribute, can be done in a Backwards compatible way by retaining the old element or value with a "deprecated" status. The depreceated value may be dropped after a while at an appropriate Major release.