There are at least two aspects to a comprehensive reply. I cannot assure you of a comprehensive reply, I am insufficiently informed. However, to my knowledge paths that are established as public access are usually quite old but new ones are established by planning authorities all the time. The width of public paths does not necessarily allow vehicular access, in fact older ones are usually based on pedestrian access or a bit more at best. It is very common to find that old paths are limited to 1.8m in width. If a path has not been fenced and the surroundings themselves do not limit the width of the path then it is possible that people have taken to driving along part or all of it even though it is actually just for pedestrians. That in itself will not alter its width in the record of ownership, etc. If the land is actually in private hands then the owner will be obliged to allow (not necessarily provide) access. If there is no stipulation of vehicular access (i.e. relatively modern) then the default will be for pedestrian only (my understanding).
The second aspect is the one about change. It is possible for the owner of the land, including local council, to apply for removal of the path and obligation to permit access. There is a lot of shenanigans around planning applications with developers and councils trading bargaining positions, an example being where the developer has to provide something that one would normally not expect to have anything to do with, say, building a few houses somewhere. The builder will be inclined to look for trade-offs as will the council. A totally different type of catalyst for change is plain and simple registration error. There are numerous cases of the seller of a property actually selling a piece of neighbouring land as being part of his and the "mistake" actually being registered and becoming official - tough for the real owner who often doesn't have a clue until too late if ever. This has even cost councils land - a poor law and worse enactment. If you don't believe this, go to the Land Registry in Edinburgh and ask them outright - if they deny it (unlikely) then they are lying. They are also the ones who can tell you definitively as to the status of the path you are looking at.