Better to assume the least favourable outcome.

I would never recommend waiting til the last minute to change any ticket on any airline, by the way; best to do it as soon as it becomes clear you have to. If you wait til the last minute, you leave yourself open to having non-responsive websites or busy call centres frustrate your attempt to get your change in before the deadline expires.

But certainly, do not use this "offer" as a reason to book a ticket which from the outset you already have no intention of travelling and which you intend from the outset to change for "free" later. You still have to pay the fare difference so ensure that you don't treat this "offer" as if it is some generous facility by which you, or any other traveller, can outsmart Ryanair and benefit from in ways unintended.