There's no need for you to stop your referral program when you change your referral incentives. Stopping your referral program prevents you from gaining new advocates during that period of time.
Your referral program's Reminder Emails will contain the newest referral incentives, and serves to update your advocates about changes in your referral program. If you wish, you can change the copy of your Reminder Email to highlight recent changes in your referral incentives.
On top of that, you can add an extra line of text in your Referral and Reminder Emails: "Note: Offers may changes from time to time". This tells your advocates that referral incentives might change.