There is outbound Avios availability for a flight I want to take using my Amex voucher, but currently nothing on the day I need to return.

I know some people book a different return date in the hope the desired day will open up and they can change the booking; I can't do that as I absolutely need to travel that day so prefer to book a fully flex single for the inbound.

So if I go ahead and book a one way outbound with the Amex, can I call and amend this to a return in the event mileage opens up for my inbound?

Thanks!