My TiVo was tuned to "The Simpsons" on channel 234 which is Fox-HD. I went into TiVo Search and look for "The Simpsons" and chose the "Watch now" link.
The TiVo switched to channel 24 which is Fox (SD). Not only that, but pressing "enter" to go back to 234 didn't work. The TiVo didn't remember the previous channel in this case. Not sure why.