You're right, it booted her back to the original seat. I forgot that she's leaving an hour later than me from ATL to MIA, so that leg really didnt matter. But we're on the same MIA to ATL flight on the return. I wanted to get her moved up since her connection time is short, and she was stuck in one of the last rows.

Before even attempting the seat move online, I spoke with a very helpful rep. I just explained my friend's short connection time, and asked if it was possible for me to do it online, since it was apparently allowing me to choose the preferred seat. She said she wasn't sure why it was doing that (and that it probably won't save). Her response: "Oh, I can do that for you. Let's see what's available..." ^