Correct propagation of process type changes to long-running process instances and the capability to perform ad-hoc-modifications of individual process instances are essential requirements for any process management software. In particular, in many cases it becomes necessary to propagate process type changes to individually modified process instances as well. In doing so, one must not only enable state-related compliance checks, but is additionally confronted with structural and semantical conflicts that may exist between process type and process instance changes. For the first time, this paper identifies and classifies semantical conflicts between process type and instance changes, and illustrates them by sophisticated examples. In order to be able to adequately deal with semantical conflicts at change propagation time we provide formal methods for conflict detection and discuss strategies to deal with different kinds of semantically conflicting changes.