JCR2 makes some fairly significant changes to requirements for versioning. It defines optional features for simple and full versioning.

Simple versioning basically only requires the addition of a jcr:isCheckedOut property to nodes and defines some basic versioning behavior based on that property. This is significantly less than the functionality currently offered by ModeShape and is probably not worth implementing.

Full versioning is similar to the current versioning capabilities of ModeShape, but it seems to add support for branches in content instead of requiring a linear progression of nodes. This probably cannot be implemented in a way that is backwards compatible with JCR1, although portions of the API changes may be able to be introduced prior to a cutover to JCR2.