This has long been a highly requested Communities feature, so the fact that it has been implemented so quickly is further confirmation that the Google+ team listens to users. To move a post, all a moderator has to do is click on its current category and select the correct category from the list.
This should help keep Communities well organized and make finding the right content even easier for Community members. It's still rolling out (I don't have the option yet), but keep watching for this feature in the next few hours!
Source: +Brian Glick