Currently Aternos does not support the ability to change the seed of a world generated with the Terralith mod. This option is separate from the seed found in world/level.dat and takes priority over it. The option for this setting can be found kubejs/minecraft/dimension/overworld.json.
The location for the seed is found at the bottom of the overworld.json file and highlighted in the image provided.
The addition of this feature would fix issues with sudden changes in world generation that come with updating modpacks that contain the Terralith mod.
Thank you for your time.