Each handler can have only one profession at a time, unless they have genius ability, but you can change professions for your rider as many times as you wish. Keep in mind that if your character will changes their profession they will lose all the benefits of their previous occupation. And if they will come back to old trade they will start from complete 0. Handlers DO NOT save old progress when you change professions!

Though professions give certain benefits in certain fields it does not mean that people who went with other professions can`t take part in activities of other professions.

For example, rider can attend street races, street racer can take part in official events, policemen and soldiers can take part in street racing and official shows. Same goes with breeders.

However, there are some restrictions:

If rider, breeder, soldier or policemen will take part in street race and will end up caught by police, they will loose one division level in their profession field (if they won`t have a getaway).

So, for example, a police detective will be demoted to officer.
(Of course in case of policemen and soldiers we speak of attending street races being off-duty. There won`t be any penalty for chasing and trying to catch nasty street racers)

No one, except policemen, can attend police quests and/or chase street racers. Same goes for Soldiers, they have their own unique quests that no others may attend.