JATS is an Open Standard. Users may modify it by adding or removing elements and attributes to suit their needs. Some publishers have extended (added to) JATS based on their own requirements. And there are some public extensions like BITS, STS, and Taxpub. Users expect significant efficiencies from vocabularies based on JATS, including the ability to intermingle the documents in databases, to use tools created for JATS for their new vocabulary with minimal additional work, and to adopt rendering/formatting applications and change only those aspects specific to the new vocabulary. Some model changes create compatible documents, which can interoperate with JATS documents gracefully. But some model changes are disruptive. We discuss what types of changes to the JATS models can be integrated into existing XML environments and which may be disruptive. We propose a set of criteria to evaluate whether a proposed change will be seamless or might cause problems.