Since nobody else has answered this, then I will.  You can't change the device button defaults with Dell's software, because Dell didn't include that feature in their software package.  Brother manufactured the printer for Dell, but Dell decided to write their own, bare minimum, software package for PC users.  As such, they didn't bother to include many of the useful features, such as the ability to change the device button defaults. 

Apple users, on the other hand, have a much better experience with this printer.  When you download the Dell drivers for an Apple, you actually get the Brother ControlCenter software, which does allow you to change the device button defaults.  I figured this out after using this printer with an Macbook for a while before switching to a PC.  You can imagine my disappointed in the PC drivers, which didn't include all the useful features that I had gotten used to on the Mac.

The Dell E515DW is nearly identical to the Brother MFC2707DW, so my workaround was to install the 2707 drivers from Brother.  At first, it didn't find the printer on the network, so I specified the IP address manually and was able to successfully complete the setup.  After that, I was able to scan using the ControlCenter software as well as change the device button defaults.  Now, when the printer asks me if I want to "Change Setting" during a scan, I select no and it scans according the settings I put in ControlCenter.  This speeds things up tremendously, because most of what I scan is in B&W which is much faster than waiting for color.  However, there are some functions, such as remote access to the fax address book, that do not work with the Brother software.