The IDMS SVC is always downward compatible. As of IDMS version 18.0 there is code that is executed at startup to access the SVC identified in the sysgen and make sure the SVC is not from a prior release.
If it is, you will receive the DC170204 message.
The way the SVC verification code works depends on a predictable load module structure.
One client was receiving the DC170204 message even though they were certain they were running the latest SVC.
What they had done was to insert a customer-specific "stamp" after the #SVCOPT which changed the offsets of Module Identifier literals that the startup code looks for.
Once they removed their custom stamp the message was no longer displayed.
Another reason you could get this message at startup even though the SVC module is fully functional is if you include an SVC Exit and the Link Edit included the modules in the wrong order.
Order should be: