How do we convert from using PROJECT_ID as a chartfield to installing Project Costing?

We are on PeopleSoft version 9.2 and have been using the PROJECT_ID as one of our chartfields for years. We would like to explore implementing Project Costing now and trying to determine the impact on our existing project id values. Ideally, we would only like to use Project Costing for some of our business units and continue to use the project id chartfield for others, but it doesn't seem like that is an option. If we turn on project costing, how is that going to affect our existing project ids that are setup under the SETID of SHARE? Also, with