When will they be available for f2p players to change their display names?

This would be a better question for the official RuneScape forums. My answer is: No, I do not think so. Being able to change your display name without any extra fees is a great feature already (for P2P) and making it available for F2P would be: 1) Too big of a load for the servers to handle & 2) Ruin the fact that it's such a privilege to be able to change your name without any additional fees (other than having to be a member). MMO's like WoW usually charge somewhere around $10-$20 (if not more) to get your name changed.

I think they said that is going to be a members only feature.

I think F2P with at least 50mil+ Exp should be able to change their names - Once A Year - at the most.

And, someone is going to throw a rock at me...

*throws a rock