Hi @PrasadWF ,
It is not possible to "upgrade" RDS CALs just by moving them to a higher version licensing server, so you have two (2) options:
1. If you have Software Assurance (SA) on RDS CALs or some kind of Enterprise or school agreement that entitles you to always use the latest version as long as you pay. In this case, you don't need to move any CALs.
2. You have bought your RDS CALs retail or via Volume Licensing, but without SA. In this scenario you will have to buy 2019 RDS CALs.
I would still strongly recommend contacting a trained Microsoft licensing specialist for this, or then talk to your licensing supplier if you can get an upgrade.
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Leon