My steps:
Import/Export Primary Key: products_id
Added:
User Defined Products Fields products_id
Exported file
Changed ONLY the Product name of 2 items and modified the model names of the same 2 products.
Saved file
uploaded
clicked import

Result:
Finished Processing Import File

Updated records: 0
New Imported records: 0
Errors Detected: 0
Warnings Detected: 0
Memory Usage: 21087176
Memory Peak: 23515400
Execution Time: 0 seconds.

Verified against the actual products online with no changes.

I did verify that "For import, the v_products_id field must be present and the admin configuration switch should be set to something other v_product_model"

Any other item I'm missing?