I'm unsure how you plan for this in advance. For one, if you do experience a delay and want to rebook, which is an "if" in and of itself, your options are based on what has availability. And the website may not offer what you're trying for. In fact, if you are delayed, an actual agent (at the airport or on the phone) is more likely to be able to force something crazy.
Two examples:
1) During a VDB in SFO, they could not get me to PHX until the following day via LAX or SLC or SEA. The last SFO-ATL flight of the early afternoon was getting ready to depart and would arrive in ATL about one hour prior to the last ATL-PHX flight of the night. They put me on that and I was able to get full MQMs, but it took the gate agent a long time pushing it through on the computer, and then it still required more work when I got to ATL. The website is NOT going to offer you this option.
2) I had an HNL-SEA daytime flight booked. The daytime flight was canceled from the schedule several months in advance of me taking it. I had already booked my island hopper flight into HNL so unfortunately, I couldn't ask for HNL-LAX-SEA to avoid the redeye, so I called DL (took two calls via HUCA) to have them force HNL-MSP-SEA. I said they forced me on to a redeye and that if they were forcing me on to one, then I wanted to be on with flat beds. The extra MQMs for HNL-MSP-SEA vs. HNL-SEA were a nice plus too. Again, the website would almost certainly not give you such an option when rebooking.

I did have a "slight" delay and rebooking via the website work out nicely once. I was flying DAY-ATL-GSO, with a 3 hour layover in ATL. There was an option with only a 1 hour layover in ATL when I booked but it was significantly more expensive. On the day of flight, I was notified that my DAY-ATL flight was delayed about 2 minutes before I even left for the airport, so I rebooked on the website and was able to move myself to the earlier ATL-GSO flight, and scored the last F seat on the flight as well. ^ But again, this is certainly not something that can be planned for in any circumstance and what you can do even if it does happen is entirely dependent on availability of other options and what the agent is willing to do for you. Best advice is if you book it, be prepared to take it.