When you create Interfaces in UML diagrams with Astah, you can choose one from 4 types directly from the tool bar.
To change this to rectangle shape, you need to do so from pop-up menu or top tool bar menu.
If you want the Interfaces in rectangle shape always, here is a trick to do so.
(1) Open “JudeP.properties” with text editor. You can find it in Userhome\.astah\professional
(2) Add this option below and save the file.
(3) Restart Astah
Then now all the interfaces you drag & drop from structure tree will be represented in rectangle shape!