As far as I can tell, there is no secure bootloader in SDK11, so it is not trivial. It depends on how much time you want to spend on it.
I guess you should look into updating the bootloader (via DFU) to the secure bootloader in SDK 12.3.0). After that, it is not trivial, because you need to change the bootloader start address in UICR. So you need an "in between bootloaders-bootloader" that does this. But it is possible. WARNING: However, if you loose power at the wrong point in time during this, you may brick the device.
I think you need to ask yourself whether this is something you want to spend time on. Is it a product that is out in the field? Or hobby devices in your home?
I gave it a go, but I am not familiar with the bootloader from SDK11 (legacy bootloader). Are you able to update the bootloader itself (to the same bootloader, just change something in the bootloader code, and try to update to this new bootloader)?
