An admin in the organization ran /PrepareSchema from an Exchange 2019 ISO and it succeeded. The following day their application vendor said Exchange 2019 is not supported for mail integration. So the admin decided to continue Exchange migration by running /PrepareAD from an Exchange 2016 ISO and it succeeded. Now I have a source Exchange 2013 server with the following AD attributes:
Exchange Organization Object Version 16217
Exchange System Object Version: 13237
The goal is to Migrate from Exchange 2013->2016.
Since there is not going to be an Exchange 2019 server, can I still proceed with installing Exchange 2016 even though the Schema has already been extended for Exchange 2019?
ETA: Per Joy Zhang's comment, I didn't consider the scenario where you may want to add additional 2016 servers to the Exchange org after running 2019
/PrepareAD
. Please consider whether that would be a possibility in your environment. My advice below pertains to an environment where no new 2010 servers were added after the prep was completed.Yes, it's fine. Although I'd run the 2016 schema prep to be sure. It's not going to "overwrite" your 2019 prep. I don't know if it'll just exit and say "nothing to do here", but it won't hurt.
Once you've got the 2016 server in, I'd recommit running the 2019 AD prep just to have everything consistent.
I worked in an environment where we had 2016 prep in situ for years while running a 2010 Exchange org. No issues.
Also, ask for vendor what their roadmap for supporting 2019, if you're on the latest version of their product. It's been more than a year since Exchange 2019 went RTM. Not that it's an excuse, since they can develop against the preview versions. Is their product unsupported if you have any hybrid? (Since you've got no choice there.) I get pretty tired of some vendors holding up the refresh cycle for no good reason - esp when there are not that many changes between versions and they just need to do regression testing, probably.
Based on my research, the supported operating system for exchange 2019 is not supported for exchange 2016. For more details: https://docs.microsoft.com/en-us/exchange/plan-and-deploy/supportability-matrix?view=exchserver-2019#supported-operating-system-platforms