If you write LE13 on the eMMC, it is not guaranteed that you will be able to boot older versions.
The Amlogic vendor kernel has some drivers that assume an era-appropriate vendor u-boot is installed so it can probably be done with a little manual fiddling (which there are no guides for) to fix wrong-named boot files, but I guarantee that some drivers don't work right, and as the maintainer of the AMLGX image; this is not something we have any interest in trying to support.