what constitutes an "acceptable" behavior of such a process. We capture this notion in our definition of the validity of an on-line change. We define an on-line change to be valid if some time after the change, the process reaches a reachable state of the new program version. Thus, validity ensures that following a change, the process starts behaving like the new version of the program after a "transition period". We first consider validity of on-line changes to programs written in sequential procedure based languages. For this purpose, a very simple model in which procedures and functions are not allowed is first considered. State is modelled as a mapping from variable names to values. For this model, we show that it is undecidable to find whether or not a given on-line change is valid. This result has important consequences. It means that computable necessary and sufficient conditions for validity of change can not be obtained. Undecidability in this simple model also