RE: All users are getting "Commissioner" role in Fantasy sales by default which can not be changed.
Hi Javed,
Are you using D365 online or D365 on-premises?
Could you provide the version of D365 currently in use, and the older D365 version you used before?
- If you are using a version after Dynamics CRM Online 2016 Update 1, please note that Gamification has replaced Fantasy Sales Team as part of Dynamics 365. Click here to know Fantasy Sales Team vs Gamification.
So you don't have to install Fantasy sales, you should install Gamification.
If you are using Dynamics 365 (online), version 9.0+, install it in "Manage your solutions" in the Dynamics 365 Administration Center.

If you are using Dynamics 365 (online), version 8.2, install it from Microsoft AppSource.
- You could refer to this user guide to understand security roles used in Gamification.
So you only need to modify the user's security role to change his role in the gamification portal. You could refer to this user guide to know how to assign a security role to a user.
Hope this helps.
Best Regards,
Lu Hao