changing permissions for existing Items (Documents)

Sure! There is even an inbuild action.

Mark your item in the grid -> right-click -> Admin -> There should be a button "Reset object access" or similar. This will change the permission of the existing item to the default permission set in ItemType.

Thanks for your reply.

I tried this in one of our test environments. There is the option called "Reset Item Access". When I use this however, the permissions do not seem to change. I also do not get any message or confirmation of anything happening.

Any idea if I do something wrong or why this would not have the expected result?

Thanks for your reply.

I tried this in one of our test environments. There is the option called "Reset Item Access". When I use this however, the permissions do not seem to change. I also do not get any message or confirmation of anything happening.

Any idea if I do something wrong or why this would not have the expected result?

To be honest, I normally don´t use this function very often. There are a couple of them in the "Admin" menu. Not sure if I recommend the correct one (I use language pack)

When I have to reset permission for multiple items, I most of the time do this directly inside the SQL database. But I don´t recommend this one if you are not familiar with the database.

Each item has the property "permission_id" that stores the linked permission. But you have to check if maybe relationships reuse this permission. 

So better check if first if you can use some of the Admin Method.