Legacy Devices are defined as 

“Medical Devices, Active Implantable Medical Devices and In Vitro Diagnostic Medical Devices that are covered by a valid certificate issued in accordance with Directive 93/42/EEC,Directive 90/385/EEC or Directive 98/79/EC and that continue to be placed on the market after the date of application of Regulation (EU) 2017/745 (MDR) or Regulation 2017/746 (IVDR).” 

Thus, the products are already on the market but still have to be registered once the new MDR/IVDR apply. An EUDAMED DI (primary identifier) and an EUDAMED ID (device identifier) will be required. Detailed information on this process can be found in  MDCG 2019-5 Registration of legacy devices in EUDAMED published in 2019. The management guide itself offers consise flowcharts and case studies.

Sources:

EC: MDCG 2019-5 Registration of legacy devices in EUDAMED 

EC: Management of Legacy Devices MDR EUDAMED