I have had a couple of conversations with Mileage Plus this afternoon. It's chaotic over there, but here's my understanding of the GS status challenge after talking to a supervisor. All current GS who did not requalify this year are automatically in the challenge. No registration required. Also, they said GS members would not receive an email about it, but I'm skeptical of that. Last year, they ultimately did send an email if I recall correctly.

The challenge is the same as last year's, except for the $1k of additional spend. Specifically, you need to spend (book and complete) $18k worth of flights on UA and/or UAX during any rolling 120-day period between 1/1/24 and 6/30/24. Once the challenge requirements are met, GS status is granted through 1/31/25.