I did a similar job many years ago and found the job much easier when the tyres were warm. I perched them on a radiator for a while and they became quite flexible and eased off without too much force. However, I thought it would be a good idea to clean the tyres and popped them in the dishwasher. They came out almost brand new but about one-size smaller (in clothes terms). I only got them back on to the wheels by heating them in water in my largest cooking pot and by wearing industrial gloves to ease them back on. Cheers, Andy