This is upgradable, do not remove your 1.3 version. And just to be sure, backup your database and the pm_attachments folder (as I always do, even for my mods... hehe). Keep it installed. Download the new PM Attachments 1.4, and add it, it should ask you to upgrade for 1.4, click to upgrade. There, all done. Everything should be fine and you should still be able to see your attachments from 1.3.