Techniques are disclosed for concurrently maintaining serialized objects that were created from more than one version of a class definition. According to preferred embodiments, a component maps and enforces multiple valid stream-unique version identifiers ("SUIDs") for a collection of class definitions (such as the classes of a software product that is undergoing change from one version to another). This component programmatically detects when an object's SUID value does not match the code attempting to deserialize the object, and then programmatically determines whether the object is compatible with the code even though the SUID values do not match. Preferably, this determination is made by consulting a stored mapping, in which the SUIDs of compatible versions have been recorded.