I don't know if there is a way to do this using the UI provided in Active Forums, but it looks like it's just a matter of changing a 1 to a 0 in the database. If you have access to the database, take a look at the activeforums_Topics table. Find the topic in question (by TopicId is the easiest) and change the value of the IsAnnounce field from 1 to 0.
This comes with all the usual disclaimers, like if your topic disappears, catches fire, or something else horrible happens, you're on your own.

I'm sure someone in here will confirm or correct me.