It's actually a bit of a complicated one, if you except the offer made by the airline then I think you have surrendered your rights under the EU legislation, otherwise according to
http://eur-lex.europa.eu/smartapi/cgi/sga_doc?smartapi!celexplus!prod!DocNumber&lg=en&type_doc=Regulation&an_doc=2004&nu_doc=261You are entitled to the following;
* 30% of the price of the ticket for all flights of 1500 kilometres or less;
* 50% of the price of the ticket for all intra-Community flights of more than 1500 kilometres, except flights between the Member States and the French overseas departments, and for all other flights between 1500 and 3500 kilometres;
* 75% of the price of the ticket for all other flights, including flights between the Member States and the French overseas departments.
This would seem to make sense to try to get someone on a different flight instead of having to pay them 75% of a J class ticket. Also AU are guaranteed a Y seat if full fare and booked 3 days before departure...