I am not sure if it is possible to change the permissions all at once (I know it is possible to view them all at once).
You are able to copy forum permissions from an existing forum, to another, though. So in theory, once you have set the desired permissions on one forum, you can edit the others and have the "Copy forum permissions" drop box set to the forum you edited.
You will have to set copy forum permissions for each of your forums, though.
To change it so guests can post on it as well, make sure your permissions are like mine:
![Is there a way to change every Forum so guests, users, members, moderators, and basically everyone be able to post on it? Screen12](https://i.servimg.com/u/f45/13/65/22/09/screen12.png)
To change permissions, follow the instructions on a previous reply I posted on another thread here ->
https://help.forumotion.com/t95366-how-can-i-allow-guests-to-post-in-my-forum#614988Hope that answers your inquiry.