How to transform transaction entries coming from legacy system to oracle with different CoA?

Hello,
I am migrating journal entries from a legacy system to Oracle. The Chart of Accounts (CoA) in the legacy system consists of only three segments, whereas in Oracle, it has eight segments. While migrating the legacy system data, I want the entries to be automatically transformed from the three-segment CoA to the eight-segment CoA based on the related values within Oracle. I need to implement this transformation for journal entries that are directly posted to the General Ledger, as well as for entries uploaded and posted from Payables and Receivables.

It would be really helpful if I could get documentation outlining the complete process for handling such scenarios.