Remember that if you cancel, it will take a few days for the credit to appear on your credit card, so that cancelling and then buying a new ticket will mean that your card will temporarily carry a double charge. If being close to a credit line is an issue for you, this could be a problem.

Otherwise, look at loads by booking class on seatcounter.com for free and pick a date to ticket now based on your guesses of
***whether the fare class is likely to sell out within the next day or two
***the most likely date for your return

Remember too that if the fare goes down by midnight tomorrow, you can cancel and reticket so as to get the lower price for either date, but be careful about any ticketing service fees. If you need to change the return from what you've guessed, you'll have to totally cancel the first ticket (losing seat assignments on the outbound) and start over from the beginning to purchase a totally new ticket. The risk-free cancellation (pay attention to the time zone that counts for this, usually the time zone of the departure city but check) doesn't include any changes, just cancellations, as others above have told you very strongly.