The dedicated 24/7 PBS HD service that was being carried on 21-1 was discontinued (by the network) on 12/17, now that the main PBS program channel is largely in HD, especially during primetime. All the programming that used to be seen on 21-4 is now on 21-1, most of it in HD - and there's now a little more bandwidth available to improve the picture quality on 21-2 and 21-3.
This was all outlined in a half-page notice on page 23 of the December program guide, and on the station website as well. (Though if someone wanted to argue that the website didn't make the transition as clear as it should have, I wouldn't argue.)
The signal now being seen on analog 21 and cable 11 is a downconverted version of 21.1, which has caused some hiccups with aspect-ratio issues that are still being worked out. (The problem is apparently at the network level, or so they tell me.)