I think you can change their name as long as they have never been raced or bred. There's a fee but I'm not sure how much.
All l I could find on the Jocky Club site was:
" A foal's name may be changed at any time prior to starting in its first race. Ordinarily, no name change will be permitted after a horse has started in its first race or has been used for breeding purposes. However, in the event a name must be changed after a horse has started in its first race, both the old and new names should be used until the horse has raced three times following the name change. The prescribed fee (see Fee Schedule) and the Certificate of Foal Registration must accompany any request to the Registry Office for a change of name. "