Thanks for looking after the disabled

I originally booked on Flight Number MN 505 at 11:20 on the 24th of June 2015. I specifically paid extra for my father having had a stroke needs to sit with as much legroom and as close to the bathroom as possible. His flight then gets changed with no warning to MN 515 at 13:50 and he gets shifted to another seat because his seat (The disabled person) was double booked.So instead of moving the other able bodied person my father gets bumped. Never mind the inconvenience caused for the people waiting at the airport to receive him, nevermind that I actually paid extra for this, never mind that your staff are both plain un-emphatic and un-educated and never mind that he is disabled we will treat our flyer like we see fit. Then I get told that I changed cancelled the ticket and changed the flight! Don't you think I would be aware of a cancellation, and don't you think he should the still get the seat as per his ticket. O' apologies your staff are to slow to understand the argument.