What they said.

Not only sweeps, but sometimes the littlest thing can cause the system to bump them out. Flight time change. Equipment change. The system seems to automatically re-look at everything when these little changes occur.

When I arrange this (not by booking and then canceling myself, but by just asking and getting an agreeable Angel), I monitor the seat assignments very closely. Many times they've been bumped back to E-minus at some point and I had to scramble to get them back up front.

Other times they just seemed to hang in there just fine.