I don't know the answer. I've done it in the past, but last time I tried it wasn't possible. However that was me trying to change a pre-Avios booking to a post-Avios one. Why not call and ask.

On the other hand, you can get around the HHA problem by opening a non-HHA avios.com account. Transfer the miles there ("Manage my account"->"Combine my Avios") and use them for whoever you like.

(If you cancel the booking which presumably came from those people's accounts too, the miles will fragment into different pots so you'll need to have ensure you have enough for the new booking, and don't mind them getting the refund.)