One of the major benefits of Application Express automatically creating a new schema for each major release of Application Express is that reverting back to a prior release is a relatively simple and low risk option. If you revert back to a prior release, any modifications made in the Application Express 4.2 instance will be lost. The main task is to switch the public synonyms and grants to point at the previous schema instead of the APEX _040200 schema. The exact steps are outlined in the Oracle Application Express Installation Guide, Appendix A: Oracle Application Express Troubleshooting section. It is also important to remove the APEX_040200 schema once tests running on the prior release are successful.