I figured out that uEnv.txt is not the ideal place for my purpose.
For the beagle bone black, it is better to adjust the uboot environment variables.
In order to store the environment, I built and installed uboot from here (which is easier than you might think, thanks to @RobertCNelson ):
in the branch v2022.04-bbb.io-am335x-am57xx , you will find install and build scripts. adjust what you need in configs/am335x_evm_defconfig and make sure, that the install script uses the correct device and partition.
Then you can use “setenv” and “saveenv” in the u-boot console persistently.
this is how i altered the variable boot_targets which is something like the boot order.
So I will boot from USB, first and this is where I put my update linux. If there is no USB, eMMC will be booted - and so on.