No need to delete any data in BW or R/3 side for enhancing datasource if loading data into ODS in overwrite mode.
Simply follow the below steps
1. Add new fields to ODS & cubes and adjust update rules.
2. Clear LBWQ(update queue - Run V3 job only).
3. Clear RSA7(Delta queue - Run infopak to pull data into bw)
4. Move datasource changes to Production, replicate and activate transfer rules.
5. Delete data in LBWG(setup tables).
6. Fill setup tables for historic data.
7. Initialize datasource if required, without data transfer(Zero initialization).
8. Pull data from R/3 to BW ODS in overwrite mode with Repair Full option.
Loading data in overwrite mode, so no problem, Just load again historic data as well. and push delta from ODS to further(ods/cube).
9. Push delta from ODS to CUBE.