“4.4.38+” indicates you have a modified kernel. “4.4.38-tegra” is what you would see from “uname -r” if it were unmodified. The “4.4.38” is the source code version, and any suffix after that is from configuration steps. So it is hard to say from what you have said what kernel you are trying to go to since both of those are non-stock for the TX2 under any R28.x release. More details on which kernel you want would help, and verify that “head -n 1 /etc/nv_tegra_release” shows R28.1.
You are in luck in terms of not needing to flash if your system is bootable. What is your current situation so far as the system being able to boot?
Even if not bootable you could make a rescue SD card, or you could clone, correct the clone, and flash the clone back in. All of this is basically some combination of updating the Image file in “/boot” and/or the extlinux.conf file in “/boot/extlinux/extlinux.conf”. Knowing what needs to be done might depend on above questions.