Just wondering if anyone knows if it is possible to change parole officers. for example, after being revoked and upson released again, can you request a new officer?

You can ask to speak to the Supervisor at any time, and see if they will assist you. You can also file a greivance. Now if you are in a large city like Toronto, you can at worst, move to another jurisdication in the city, thus automatically you'll have a new parole officer.
There is also a really high turnover for parole, so you might just luck out.