The Find and Replace function in Project Client has the field of "Item permissions" so you could load your items into Project Client and use this to batch change permissions. Here are the steps:
- Pull the items with permissions you want to change into a Project Client project.
- Select the records to which you want to make the change.
- Click the Find and Replace button in the upper left above your collection name.
- Type the current permission information into the Find what field.
- Type the permission you'd like to replace it with in the Replace with field.
- Select Item Permissions from the in drop-down.
- Click Replace or Replace All to update the record permissions.
- Upload the edited records to send up the changes.
There is more on editing item-level permissions.