Hello Mark,

A safe way to approach this is to perhaps make a copy of the Form Business process, and make the changes there. Then have your users use that new process to start new processes. That will allow the old form business process to finish and prevent any sort of interference.

To answer your question however, making changes will not corrupt it, nor should it cause any real issue. But depending at which steps certain instances of the Forms process is at, it may use the old model, rather than the new one.

For example, you have an instance on Step User Task A, which is currently waiting for user 1 to approve a form. If you change User Task A to have the Form to be reviewed by User 2 instead, the current instance will not change it to User 2. It will still remain on User 1. Only instances that have yet to reach that User Task A or initiated after that change will send the approval pending to User 2. 

Hope this helps.

Andrew