I haven't yet found a way to 'mass convert' track from one type to another (ala, standard gauge to 42" gauge etc) - although I presume editing a route's config file by changing content in it's KUID list would be possible.
I have no idea how effective that would be though (and may cause errors to appear in the route too).
So basically the only way would be to open a route in Surveyor and manually delete track and then relay it from scratch. A monotonous and time-consuming task.