There’s a good description here: Flash partitions
I’d just emphasize that this is a dangerous operation. As @amitchell mentions, it’s destructive and erases partitions if the start address changes, or size is reduced. It’s also very important that you don’t reduce the LK or Kernel below what you need. The AT command will not be available to revert the issue, if the system no longer boots after the change. Your plan to increase rootfs and decrease user sounds safe though.
As you allude to, LK is providing this info to the kernel. But if you remap the partitions there, Sierra CWE download will no longer work for the affected partitions, because the original partition table, used for flash programming from SBL (System Bootloader) will not be updated.
These considerations are the reason the command is not currently supported on the WP76xx/WP77xx variants.
Ryan