If you create a new business process for your opportunities in CRM, the records that already exist do not update automatically. It may appear that you have to update each individual record to use the new process but there is a way to do this in bulk. Once you have your new process set up and your old process deactivated, you'll start seeing a message like this on your existing opportunities. This is a good reminder that you need to switch the business process.
If you have several opportunities to update, you can do it all at once using a simple view. Create a view of opportunities that includes the field Process.
In Excel Online before changing the Process ID:
Then import your changes and all of the opportunities will now be using the new process!