How to add new variants to an existing product template?
Let’s say you have a product with 2 variants initially and you want to add a new attribute with 2 values so that your product will have 4 variants from now on.
By default, Odoo will delete (or archive when already referenced) your initial variants and create new ones.
If you want to prevent this, then first add your new attribute with just one value. Now Odoo will apply this new attribute value to your existing variants.
Add extra attribute values in a second step and now new variants will be added to your product template without touching the old ones.