If your current ty
re is cracked then it may just break into pieces when you try and roll it off the wheel - I've had 2 do that to me - otherwise you could try heating it with a hair dryer which should make it more pliable to roll off.
When you get the new one just drop it into a cup of hot water for a few minutes which will make it soften and then roll it on to the wheel with a strong push.
New tyres and lots of other things can be bought from Helen Howes website in the UK if you have no joy else where -
http://www.helenhowes-sewingmachines.co.uk/winders.html
Anneg