The idea is the following:

"we use a binary persistence format (using a byte[]) to transform a process instance or the session state into a binary to put it in the database. It is possible however that this binary serialization algorithm might change over time (when new features are added etc.).
...
we are gonna have to make sure that, if a user updates from one jBPM version to a newer, that the old binary array can still be read in (or transformed).

The idea is that we should set up some infrastructure to keep track of this. And when we do encounter issues, we can then see how we can solve these (we're thinking about the ability to plug in custom transformers that are able to translate the binary array from one version to the other). Ignoring the second step for now, if we want to be able to detect issues, we should at least have some binary session and process instance info objects for each version (starting jBPM 5.1) so that we can run them against future versions."