Most definitely not.
I wonder if you realize that u-boot and the kernel are actually on your sdcard?
Libreelec distributes mmc (i.e. uSD/eMMC) images. These images contain all the related pieces, like u-boot, kernel, etc. If you install debian and then reinstall libreelec, then the libreelec image will overwrite ALL of debian on the card.
You should also put some more thought into what your goals are. For instance, WHY do you want to change the u-boot and kernel on libreelec? The pieces of libreelec are selected and configured to fit together and actually work. Try changing out the kernel with the one from debian, and it probably won’t even boot, let alone work the way you want it.
If your goal is to experiment with bleeding edge kernels, then libreelec is not the best choice of userspace. In fact, you could very easily install kodi ON debian to get yourself libreelec like functionality while satisfying your need to spill blood. Libreelec is made to “just work” (ideally).