Any way to test bulk upload price changes using a copy of the real site?

Site URL: https://www.phelpsrefinishing.com/

Last December I tested the whole bulk editing process on a copy of my client's site, so I could be sure the same procedures would work on January 1 when every price had to be updated. Now I'm in the same process for an August 1 update, and have not succeeded yet in making any test imports work. Of course the test site uses different SKU numbers, so I export the test site inventory, make a test change, and import it back to the site. The import result is just duplicate products with completely new SKU numbers -- no change to the products displayed on the test site. 

It worked so well last December that I didn't make many notes on how I did the bulk editing...except for notes on how to get the variants updated. Tech support hasn't solved this with me yet. Anyone else have ideas on what I'm doing wrong?

Meanwhile, I'll keep trying different things and if I find a solution, will post it here.

I could never get the test site uploads to stop just making duplicates with new SKU numbers.

So I duplicated the real site again, and selected 3 products from the same page to export. Tested importing those back -- worked fine. Changed prices on one. Worked fine.

That's a baby step toward uploading a whole page, but it's encouraging. Images look fine, no duplicate SKU numbers.

Tech support had told me that my problem was that I hadn't used the format of their upload sample csv, clearing the contents of the Product ID, Variant ID, Product Page, and Hosted Image URLs -- just keep those column headings. I stopped getting rejected uploads but of course that's when the upload created copies with new SKU numbers and usually, no image.   

The new uploads of the 3 products didn't clear any column contents at all, so I'll keep testing more elaborate spreadsheets tomorrow.