Prevent user from file overide

Hi,

I would like to prevent our workers from being able to overwrite an existing file, instead forcing them to have to do a 'Save As' process.

So they should be able to open a file but only able to save it as a new version name, not being able to overwrite the existing file they have open.

Make sense?

I thought this would be possible by deactivating the 'Modify' permission for that user group but it prevents them from saving any file. They need to be able to save a file but just not with the same name as what already exists. This is to prevent them for saving over versions we might want to go back too.

Many thanks,

Richard