Don't know if I just was impatient or if it helped, but when I tried on mobile instead of desktop, I was able to change it. (I was at 11 posts at that point.)
Leaving breadcrumbs here for the next newbie.
To answer the question - you have to have a minimum of 10 posts (not including any posts on DVC board) AND be registered on DISboards at least 24 hours. The time for the "system" to recognize that you've achieved both requirements can last up to several hours before giving you the privilege, although, typically, it's about a hour.
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