No, I don't have any stats as it is more a case of observation over many years. All the top stables do it and most of the top jockeys now have managers to procure rides for them, so they are on the lookout for prospective city winners.
You can see the strategy-an apprentice or unfashionable rider gives the horse a good hit-out then when the horse is primed a perceived "more competent/fashionable" jockey is engaged. This strategy has worked at the provincials for years and will continue to do so as most of the top hoops do not frequent the provincials regularly. If they do, especially for only one ride, it is time to take notice. One of the top hoops is only engaged when a horse is considered good enough to be brought to town. It is just another factor in your form study and I just offer it for your consideration.

Cheers.