Yes, this is a feature they've been deploying for the past several months. It's been available via web check-in for almost a year now as well, but they started pushing it to the mobile apps sometime around November or so of last year.

It's of limited use. Generally all window/aisle seats are already taken if you're on the upgrade list, meaning that you have to hope for two things:

• That enough people either no-show or get upgraded into F for window/aisle seats to clear AND
• The gate agents actually use the Comfort+ upgrade list instead of just clearing standbys into C+ (which happens anywhere from 50-80% of the time based on my experience)

On some routes I'm "lucky" enough to frequently be #1-#2 on the C+ upgrade list and I clear maybe 20% of the time.