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We wanted to write an article related to a very frequent customer question related to Exchange Server 2019 to Exchange Server Subscription Edition (SE) migration.

As we mentioned in Upgrading your organization from current versions to Exchange Server SE – there are two ways to migrate to Exchange SE, when released:

• Traditional (legacy) migration. The “add a new Exchange server to your organization and migrate mailboxes to it” approach. This is mandatory if you are moving from Exchange 2016 to Exchange SE, but is optional if you are moving from Exchange 2019 to Exchange SE.


• In-place upgrade. The “Install Exchange SE Release to Manufacturing (RTM) build on top of Exchange 2019, upgrading your Exchange 2019 servers into Exchange SE servers” approach. This is new and is the approach that some customers see as “too good to be true” and possibly risky.

Let us explain why we view this second approach as “low risk”.

How RTM of a new Exchange version used to be created

In the past, when a new version of Exchange was released, we would make significant changes to the code base. The following is a simplification, but it gives you a general idea. Usually we would:

1. About two years before the release of the new version, take a version of the current in-market product as “base code” of the new product


2. Add and remove a bunch of features


3. Change installation prerequisites


4. Set new minimum hardware requirements


5. Create and require new product keys


6. Change the “major version of code” in our build system (you can see this change from “15.00” in Exchange 2013, to “15.1” in Exchange 2016 and “15.2” in Exchange 2019 as per this page)


7. Close to the release date, port over any significant features, security or hotfix updates released for the current in-market product in last 2 years (because Step 1 above was done long ago)


8. Change the product name and update the EULA (End User License Agreement) – which is a file in the Exchange image

Because many of those changes were significant, this essentially required customers to do the following:

• Perform a legacy (side-by-side) migration.


• Do a significant amount of testing in their organizations to ensure that added or removed features do not interfere with their business processes or 3^rd party applications.


• Develop training for end users (because of significant product changes).


• Wait for CU1, because RTM build was seen as “high risk” as many features were added / removed. (Note: waiting was never a requirement, but we know many customers would wait for at least CU1 before migrating).

How we are creating Exchange SE RTM

As opposed to the long list above, Exchange SE RTM is being built as follows:

1. Start with Exchange 2019 CU15 (our most recent CU) as “base code”


2. Add any security or hotfix updates publicly released since CU15


3. Change the product name and update the EULA (End User License Agreement) – which is a file in the Exchange image

Note that the first two things above happen during release of every single Exchange CU. Exchange CUs are always “cumulative” and contain all of the fixes released since the “last CU” (read more here). Therefore, the only new change in Exchange SE RTM is “3. Change the product name and update the EULA”.

We are not doing any the following in Exchange SE RTM, compared to Exchange 2019 CU15:

• Changing the major version of code


• Adding / removing different features


• Fixing any bugs that are not already released as updates for CU15


• Adding / requiring new product keys


• Changing installation prerequisites or hardware requirements

Because of all of this, installing Exchange SE RTM on your Exchange 2019 CU14/CU15 server is essentially installing Exchange 2019 “CU16”, but we changed the name of this CU to be “Exchange SE RTM”.

Why upgrade then? What is the point of all this?

There are several ways in which we believe this approach takes the time pressure off your organizations:

• It is a simple “no significant changes since Exchange 2019 CU15” installation, so there is no need for extensive testing and validation.


• By upgrading to this Exchange 2019 CU16 SE RTM build, you switch from the Fixed lifecycle policy of Exchange 2019 (end of support life on October 14, 2025) to the Exchange SE Modern lifecycle policy and your product is supported after October 14, 2025.


• Even if you want to upgrade your Exchange 2019 hardware for Exchange SE, you can in-place upgrade to Exchange SE first on old hardware and then, once you are on the new support lifecycle path, you have the time to upgrade your server hardware.


• Feature work for Exchange SE will start in CU1; as your organization can be on the “low risk” but supported RTM release, you will have longer time to test CU1

Conclusion

We realize that with this approach to Exchange SE RTM, we are doing something different, and we probably did not articulate enough just how different. As far as engineering work is concerned, Exchange SE RTM is a re-branded “Exchange 2019 CU16” with no features. We released 15 CUs for Exchange 2019 already and have a high degree of confidence that this can be a very low risk action for our customers.

To recap:

Exchange Server Team