Interesting, so here's what happened in my experience...
I was reading through this thread and saw about National Geographic in HD which interested my wife immensely, and that theme also has Showcase and TLC, so I went online and sure enough the only choice would have been to uncheck my current (legacy $10 HD) subscription. So, I called Bell instead. The young man wanted me to switch to an $85 package where I could have all the HD themes and get one more SD Theme package, but this would have been $14 more than the $76 i am currently paying (as it did not include a couple of picks I do separately - TBS, etc.) - and all I wanted was the extra HD theme. I asked if I could not just pay the extra $5 and he had to "clear" that one with someone, but came back and said yes, I could. Short story - I got the extra HD Theme pack with NG HD, TLC HD and Showcase HD for the extra $5. a month and keep all my other programming.