This page tracks the status of R2 <-> R3 transformation efforts.
FHIR Mapping Language
The FHIR specification itself includes [FHIR mapping language transforms].
Todo: updates to these can be found ....
Java reference implementation
The Java Reference implementation has the Java Class org.hl7.fhir.convertors.VersionConvertor_10_20 which has conversions for many resource types.
Note: the only actively maintained resource types are the conformance resources - these are maintained and tested for the Implementation Guide publisher. Other resource types are less well supported.