Recommendation for BPM process definition version usage

[sarveshmvm]

Hello All,

I do have a process definition which has various steps and interacts with many ecternal systems. Whats the best recommendation if I have to update the process with set of new changes and I know the old instances will still point to the old processes. We can always run a new process with old process version id but can we update the old processes with new process changes? Any inputs will be appreciated. 

I am using activiti version 6.0 .

Thanks