Convert public file fields to private

Greetings, almighty All.

(Dripal v7.18)
At the beginning I created a file field for my content type and made it "Public" (upload destination). It seemed ok that time, but project evaluates and now it is required to change this field to "Private". I have around 2000 nodes with the file uploaded to them in this field. Also this field is in 2 field collections.

How can i do this?

My guess is to do something like this:
1. Move files to "Private" folder
2. Change Field setting (may be in MySQL table)
3. Change paths for every node (may be in MySQL table)

But how exactly to do it? May be someone already done it?

Any ideas are much appreciated.