A technically supported Windows Server upgrade can still disrupt RDS if the licence infrastructure is left behind. CAL versions are not forward-compatible with newer Session Hosts, and an older licence server cannot process a CAL from a later Windows Server release.
Microsoft documents RDS CALs as either Per User or Per Device and requires an activated licence server for RDS Session Host deployments after the applicable grace period. Always confirm current terms for your agreement and deployment with Microsoft or your licensing provider.
Match the CAL to the Session Host
Microsoft’s current compatibility guidance shows that an RDS CAL can access a Session Host of the same or an earlier version, not a later one. A Windows Server 2025 Session Host therefore needs 2025 RDS CAL entitlement.
Upgrade the licence server in the right order
The licence server must be the same or newer version than the CALs it processes. Microsoft recommends upgrading RDS infrastructure components, including licensing, in a planned order before newer Session Hosts depend on them.
Test more than connection
After the change, confirm the licensing mode, specified server, CAL pack visibility and RD Licensing Diagnoser status. Test new and existing users and keep the old environment available until the licence path is proven.
A practical checklist
- Inventory Session Host versions
- Record installed CAL pack versions
- Check the RD Licensing server version
- Include licensing in the rollback plan
Licensing reference: Microsoft Learn: RDS Client Access Licences and licensing Remote Desktop Session Hosts. Product terms and agreements control; confirm the current rules for your organisation.
The practical conclusion
Treat CAL compatibility as an upgrade prerequisite. Resolve entitlement and licence-server versions before the Session Host change, while time and rollback remain available.
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