The documentation has wrong information about updating process:
https://access.redhat.com/site/documentation/en-US/JBoss_Enterprise_BRMS_Platform/5/html-single/BRMS_Business_Process_Management_Guide/index.html#Updating_Processes

There is mentioned:

"The updated process must have an updated version number to distinguish it from the old process; this is necessary because the old process may still be required by an existing process instance. "

However it is not enough. To update a process is strongly recommended also update the process definition id. So, the correct sentence should be:

"The updated process must have an updated version number and a new process definition id to distinguish it from the old process; this is necessary because the old process may still be required by an existing process instance. "

I think that it will be better if included how the routine to update a process looks like, example:

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The procedure is:

1. model process with id process.v1
2. start process instance with process.v1 as id 
3. model new version of the process with id process.v2 usually as a copy of process.v1
4. when you start new process instance you’ll use process.v2 as the id
5. continue working with active instances of process.v1

With that you have clear separation between process definitions that gives you option to run different process instances with different definitions. Although there might be cases where you would like to override the definition for the active instances and thus not changing process id gives you this possibility. 
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