Hello,
Based on version compatibility, you cannot use an earlier version of RDS CAL to access a later version of Windows Server. If you have an RDS CAL for Windows Server 2016, you cannot use it to access Windows Server 2019. You may need to purchase an RDS CAL that matches Windows Server 2019.
(Image source: License your RDS deployment with client access licenses (CALs) | Microsoft Learn)
I hope this helps.
Regards