Hi Ihor

In addition to the sequence of prompts when changing the attachment
directory:

1. Copy over attachments from old attachment directory to new one?
2. Delete old attachment directory?

we can introduce a custom variable `org-attach-move-attachments' with
the following values:

1. 'ask (default) - display the prompts
2. 'move - move the attachments form old to new directory (same as yes,
   yes answer)
3. 'copy - copy the attachments, keeping the old directory (yes, no answer)
4. 'keep or nil - keep the attachments in the old directory (no, no answer)