Recurring Campaigns will process multiple times based on configuration. For example, if the campaign is set to begin on a Monday and run weekly for 4 months, then it will run every Monday for 4 months.
If you decide to adjust the End Date of the Recurring Campaign to run for a longer or shorter period of time the campaign is unable to be adjusted by changing the End Date.
An error will be presented informing you that it can't be adjusted because the Start Date as passed.