B4R Question Arduino Mega 2560 with Ethernet Shield: boot sketch from SD-Card

Hi there,
I spent a while searching the forum but didn't find hints targeting my question: I want the Mega to boot from a hex-file from the SD card.
While it is obviously possible to transfer a compiled sketch to the Arduino Mega via an Ethernet connection, what is necessary to reset the board and make it to boot from a new program file from the SD card?
Eventually, I'd like to have both options:

• Upload compiled sketches to the Arduino the ordinary way from the IDE via USB cable
• Arbitrary reset the Arduino (by program) to initiate a boot from a previously provided file on the SD card

What I realized so far is that I would need to modify the default bootloader ... but what I read about that are just fragments I can't connect. Has someone already accomplished what I try to do or can at least point me in the desired direction? Hope for your interest and ideas - thx in advance!