EXCPDEF dataset is the exception dataset used for Exception. When upgrading from one release to another, you can either choose not to copy the previous Exception definitions across when creating the Exception dataset through the post-install panels OR you can create a completely new Exception dataset using the IDVSAMSY member in highlvl.SOURCE (NOTE: the member also has the definition for the PROFILE dataset, so that should be removed prior to running).
Recommendation is-- everything should be at the same maintenance level. Reason being, there may have been changes to dataset content being release specific. i.e. If you generate r19 jobs into the R17 .SOURCE library, it may overwrite existing JCL. You may reuse the .PROFILE dataset, the .EXCPDEF and .SOURCE libs but again- everything should be at the same maintenance level. In addition, mixing release libs would be difficult to maintain.