Will disabling existing comments delete old comments

Yes, the plugin does keep old comments.

But the only way to disable comments on current posts/pages/etc… is to click on the button on the settings page that will disable them on all of a certain post type, which is not reversible. If you just want them all off, and later all on, then it will work, but if you have a mix of some pages disabled and others enabled, then you may want to look at using a different plugin.