I had a EWR-MEM-MLI booked a couple weeks ago and wanted to standby on an earlier EWR-DTW-MLI routing. The EWR folks were extremely reluctant to do the reroute, but I convinced them to let me try. It wasn't exactly clear to me whether their reluctance stemmed solely from the sketchy availability on DTW-MLI, or whether they were actually bending rules to let me reroute. I didn't ask too many questions.

In my case I got extremely lucky. Not only did I snag the last seat on DTW-MLI (thanks to someone else's misfortune in missing their connection, according to the GA at DTW), but my original EWR-MEM flight ended up getting cancelled due to weather, so I wouldn't have made it to MLI that night at all had I stuck with my original flight!

My advice:

1) Be nice to the agents (that should be a given, but reminders never hurt)

2) Keep in mind that it's a gamble: though seats might be available on the connecting flight when you first check in, other pax can sneak ahead of you with higher stanby waitlist priority (e.g., if you choose to go standby, and somebody else simply misses a connection, they'll be higher on the standby list even if you checked in before them).

3) Keep your BPs to make sure you get WP credit. When tickets are getting reissued, WP numbers sometimes get knocked out of the reservation.

Good luck!