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Is it possible to change contribute permission for assignees in bulk?

Resolution

Admins can define permission to work work items during the upgrade process. 

Another option is to modify project preferences. Making adjustments here impacts all projects both current and future. 

The quickest option would be to select tasks assigned to certain user and change the permissions from the bulk sharing screen and perform the same action for other assignees.

All of  these methods are available to aid in modifying permissions.