First of all, you should not use persistent mode if you only want to disable comments temporarily. Please don't ignore the warnings that the plugin gives you when you enable this option!
If however you do use persistent mode and then decide you want to enable comments, it's not difficult to do. Either:
- Bulk edit your posts. From the Posts Screen, check the boxes next to all the posts you want to edit, select
Bulk Actions -> Edit, and click
Apply. In the bulk edit area that appears, check the box for comment status and click
- Edit each post individually. Go to the
Discussion box, and then check the box to enable comments. If you don't see the
Discussion box, click on
Screen Options at the top of the screen, and make sure the
Discussion checkbox is checked.