> Is it possible yet to change the visibility of old posts? The absence of small privacy features like this—features that FB does have...

Are you sure you have thought this through? Allowing retro-active changes to visibility (unless it's only in a more restrictive direction) can be a privacy violation for the commenters, who may have commented based on the original visibility list. And allowing visibility changes towards a more restrictive set will confuse the people who could see the post till yesterday, might even have commented on it, and suddenly can't see it anymore.

Privacy decisions are not as simple as they might appear on a cursory glance, and overall I find that G+ takes privacy very seriously. Seen from the above light, the current policy of not allowing visibility changes, but allowing the author to delete the post, makes a lot more sense.