Hi,
You can change the name directly via the menu Products > Characteristics.
To add new values, you can use the mass actions if the variants have the same price than the main product.
If they have different price, it will be more difficult.
So if it is the same price, you can create a mass action to remove the desired characteristic, and then use an action to add the characteristic with the new values.
Add limits, else with 10000 products the server will not support the charge.
And don't forget to make backups before anything