When I switched doctors, I had the records from my previous doctor sent to the new doctor. Also, every test I ever had, I asked for and received a personal copy.
Transfers of medical records may not mean that ones former doctor would erase his history with any patient, but would retain original or copies of records, due to liability questions or issues.
After all, former doctors do not transfer or defer their liability regarding their patient services, diagnosis, treatment & prescriptions, etc.