Process versioning

Process version execution

My question is about process versioning with Bonita.

Let's say I have a process with four activities (A, B, C, and D), and I create two instances of that process. So, when I change the process to another version with slightly different activities (A, B, D, and F), what happens to the two instances I previously created? Do I need to migrate them to the new process? Or do they become invalid?