Cannot Upload Variants for Existing Product

Overview

You are trying to upload new variants in bulk for an existing product and encounter the following error:
All variants in a product should have the same attribute keys

It is not possible to upload variants for a product that is already created.

Solution

1. Upload new variants while creating a new product (you can create a dummy product that will not be used further).
2. Submit a request to Symphony Support to remap the newly uploaded variants to the desired existing product, including the IDs of the variants and the ID of the product to which the variants need to be mapped.