I've been able to get a bump before going to the airport once, and it was because DL was being pro-active, not me.

I was scheduled to fly BMI->ATL in the afternoon. I received a call from DL in the morning (8 hours before the flight) saying that they were looking for volunteers. I accepted a $300 voucher and confirmed first (on all segments with 2 cabins) on a routing going thru DTW instead of ATL that left about an hour later.

I have heard that this was just a trial process DL was testing (probably to see if calling ahead could save some voucher money by taking volunteers ahead of time). It only happened once to me, and I don't think DL will give you a voucher if you call them.