Is a way to reassign the manager of an opportunity in bulk?
Although there is no actual option to assign Opportunities to other users en-mass you do have the ability to assign the activities of one user to another user via the territory realignment feature, this does alo include opportunities.
1. Tools > Data Management > Territory Realignment
2. Skip through the wizard to the Activities section
Here you can reassign the activities of user 1 to user 2
NOTE: This process cannot be undone, so we request that a backup is made prior to performing this task.
Also there is no way of reassigning just the activities linked to the opportunity to the new opportunity owner, so be careful if you wish to reassign the activities as well as you will reassign ALL activities and not just the ones linked to the opportunity.