I think I can answer my own question. I found the letter that Charter WI sent in late December. It says :
Dear Charter Customer, Effective on or after January 30, 2013, Charter will offer the following channels exclusively in a digital format. This change will require digital equipment in order to view these channels on all of your television sets.
Charter Media 20
Home Shopping Network 21
WVCY- IND 24
NBC Sports Network 31
Spike TV 34
Travel Channel 46
Bravo - East 63
To help you with this transition, we are providing you 2 digital receivers or 2 HD receivers or 2 DVR boxes at no cost for 24 months.*
So these all moved to unencrypted QAM, so no proprietary tuner is required. (Where they moved is another question. Time to re-tune your sets.)
I note that the BTN web site and Charter's own channel line-up web site still show BTN at 50 and 664.