Hello Tony,
I think by changing the Segment Release in IDOC type and Segment Application Release on Partner profile, SAP should be able to pick only that segment version and send out the information. You may not need to cancel the release.
Creating an Outbound Partner Profile - IDoc Interface/Electronic Data Interchange - SAP Library
12. The segment release specifies the release from which the segment definitions (not the IDoc type definition) originates. We recommend that you leave this field blank so that the most recent segment definition is used.
I think, yes when there are mix of segment released they would be ignored. I don't have specific use case to validate that, but from help:
If a release is entered in the field, the segment definition that was released for this release is used for every segment type from the current IDoc. After an upgrade, the same segment definition is output by specifying the release.
Can you try maintaining both here?
Segment Release = 700
Segment Appl. Release = 603
I think you can ignore the conversion error and see if your 3rd party is able to handle the new IDOC.
Regards,
Naimesh Patel