Here are some tips on using the Change Owner flow step when you are integrated to SFDC:
A record must be synced to SFDC
Leads can be changed to Users or Queues
Contacts can be changed to Users
Contacts cannot be owned by a Lead Queue. If you assign a Contact to a Queue, nothing will happen.
The change owner flow step will only change the lead/contact owner. It isn't configured to change account owners.
If you use the Sync Lead to SFDC flow step and choose the auto assignment rule, if the lead is already in sync and assigned to an owner it should stay the same. The lead owner will not change. You need to use the Change Owner flow step to assign the record to a different owner.