Wrong oil in megan auto gearbox?

1999 Renault Megan 1600 RT, auto gearbox re-filled with Renault D2 oil after draining old oil, sold wrong oil by Renault dealer, im now told bring your car in for a free oil change as it's our mistake, we should of sold you D3 gear box oil, gearbox hangs onto the lower gears longer thereby useing more fuel,( this problem has been progressivly getting worse) would this have done any damage to the workings of the box, this took place 12 plus months ago and the mistake has only just come to light, there's 3 .5 lt on oil change but the box holds 6 lts im told , so i think the 2.5 ltr that can't be changed on a normal oil change will still be in the box and contaminate the new oil, will this compound the problem?, im not looking to make a silly claim against the company but would like my car back to how it was prior to this all taking place without any more expense to my self, hope you can help, Regards Dennis.