US Airways Dividend Miles (Pre-FlightFund Merger) - Questions about Silver/Gold challenge

I want to sign up my girlfriend for the Silver/Gold challenge. She will definitely be flying enough in the three month window to make the Silver challenge, and may fly enough to make Gold. My question is-- will she become Silver as soon as she flies 6250 miles and then become Gold after 12500, or if she wants to go for the Gold challenge will she have to sign up for something seperate and get no status until she reaches 12500 miles? If the latter, then what happens if she ends up at the end of the three months with between 6250 and 12500 miles? Would she then be Silver?

I called last week to ask about a challenge for gold. She said there wasn't any challenges right now but there should be one in the fall.