They told me that fares within my fare class were available and that there was no fare difference.
And anyway the terms and conditions of my original ticket would stand by law. If they had told me my fare class wasn't available that is a different story and would be my tough luck.
The terms and conditions at the time I bought the ticket said that there was a free stopover allowed in each direction and that a date change would cost £100 GBP.
They wanted £100 date change, £100 stopover fee plus stopover tax (around £20).