Do you mean "fallen angel" during aviation training? If you drop during flight training, you are entirely subject to the "needs of the Marine Corps" at the time.
If you mean dropping at TBS, the policy changes periodically. I'll leave it to the current newbies on that one. When I went 10 years ago, if you dropped your contract, you would compete with everyone else at MOS time. I'm told that has changed since. You can't, however, keep your flight contract, get a ground MOS, see if you like it, then drop out of your air contract.
As to your last sentence, ground contracts can go for air, but the available slots are VERY scarce for them at TBS.