As soon as you try to rebook to a date outside of the +/- 14 days permitted period you will be charged fare difference and a change fee if you rebook yourself online. You should use the chat feature on the webpage and let a customer service agent change your travel date. This is how I have done it numerous times this year following FR cancellations. Eventually you will get through the queue to chat with an agent so just be patient.

Despite the permitted rebooking period stated in the cancellation email from FR, most agents have no issues rebooking you to a date outside the +/- 14 days window; if an agent refuses to rebook you for free, end the chat and try again with a different agent.