Why your old server CALs become worthless when you upgrade

In brief: Windows Server CALs are backward compatible, not forward compatible. A 2025 CAL can access a 2016 server. A 2016 CAL cannot access a 2025 server. If you only factor in the server license when upgrading, for 25 users, you typically forget half the cost.

The rule in one sentence

The CAL must be at least as new as the server it accesses.

That's it. The rest are exceptions and details, but if you remember this sentence, you won't make the expensive mistake again.

Why this is the case

Microsoft links access rights to the server version, not the client version. If you operate a Server 2022, every person or device accessing it must have an access license for Server 2022 or newer. A 2019 CAL is not sufficient, even if everything works technically. There is no block, no activation error, nothing to stop you. The violation only becomes apparent during an audit.

Conversely, the situation is uncritical: anyone who has purchased 2025 CALs and still has Server 2016, 2019, and 2022 in their inventory is covered everywhere. That's why larger environments with mixed inventories often buy the highest version for everyone at once.

The only real exception

Software Assurance. Those who have SA on their CALs and keep it active receive new versions without having to repurchase them. That's exactly what SA is for. If SA has expired, this right is not lost retroactively, but they will not get the next version.

The same applies to Subscription CALs as long as the subscription is active. If it expires, the access rights are lost.

What this actually costs

An example calculation for a company with 25 employees, each working from a laptop and a desktop. We sell server cores in 2-core packages, access licenses in 5-packs – both are typical units in volume license trading:

Access licenses account for almost half of the licensing costs here. With 60 employees, the ratio shifts significantly in favor of CALs.

Note on core minimum: Microsoft requires at least 16 cores per server, regardless of how many cores the hardware actually has. Eight 2-core packages are therefore the smallest permissible order for a physical server.

The second mistake: RDS CALs are on top

If you use Remote Desktop Services, you need additional RDS CALs. Not instead of the regular Windows Server CAL, but in addition to it. Each terminal server user therefore needs two licenses.

RDS CALs are also version-bound and not forward compatible. And they are by far the most expensive part of the stack: A 5-pack of RDS User CALs for Server 2022 costs €454.95 compared to €119.95 for the regular User CAL – almost four times as much.

User or device: the question before the question

Before you count, decide on the model. The rule is simple: count both numbers and take the smaller one.

• More devices than people (office, everyone has a desktop, laptop, and mobile phone): User CALs are cheaper.
• More people than devices (shift work, workshop, cash register): Device CALs are cheaper.

A company with 20 employees using 6 shared computers will pay less than a third with Device CALs.

More on this: User CAL or Device CAL: which access license you need

What you should do now

When a server upgrade is due, note down four numbers before the first quote: number of physical cores, number of people with access, number of devices with access, and whether Remote Desktop is involved. With these four values, the license calculation can be done in five minutes. Without them, it cannot be done at all.

The CAL configurator on our guide page calculates the combination directly. You can find the right access licenses under Access Licenses, and server cores under Windows Server.

Related: Windows Server 2016 End of Support: a cost comparison of the four options