You can change them in bulk through Multi Item Actions following this process - https://www.youtube.com/watch?v=Zyc-P1ZC4Z0. Outside of that, you could accomplish it by using CSV using the process below.
You'd have to follow these steps (Please note: Excel is not recommended as it will have a tendency to change all of your ebay item numbers in the reprice sku column to scientific notation and if left like that, skugrid cannot read it once you import the CSV back. It is recommended that you use Libre Office or Google Sheets instead which will not do that to your item numbers. Both of which are free to use)
1. Export your CSV from Sku Grid by going to Import/Export --> Export CSV --> Click the link to download the CSV...note: if you have more than 15,000 items, you'll have to enter an email address and wait to receive your CSV via email). Do File --> Save As or File --> Make a Copy and save the CSV file to your computer with a new name.
2. Delete all rows that do not have a Walmart URL in the vendor_url column.
3. Put your mouse in the first vendor_url field that has data containing a walmart URL and drag the mouse down to highlight the rest of the URLs.
4. Copy the URLs
5. Visit http://textmechanic.com/text-tools/basic-text-tools/merge-text-line-line/
6. Paste the Walmart URLs into the Input 1 box
7. In the Suffix Box, type #skuopt:walmart
8. Click the Blue Merge Lines Button below. This will "concatenate" or add the suffix #skuopt:walmart to all of the URLs you pasted in.
9. Click the Blue S button to select all of the resulting URLs
10. Put your mouse in the first vendor_url field containing data and paste (Ctrl+V) the URLs.
11. All of the URLs should be replaced and now have #skuopt:walmart at the end of them. Save your revised CSV.
12. Copy all of the ebay item numbers from the reprice_sku column
13. Go to your Sku Grid account and go to Batch Options --> Remove Items By Ebay/Amazon ID.
14. Paste in the Item Numbers into the pop up and click the blue Remove button. This will remove the 5000 walmart items from your skugrid account. It will take some time to process.
15. Go to Import/Export --> Import CSV
16. Browse for your Revised CSV and upload it.
17. Only the revised items will be uploaded. It will take time for them to fully update in sku grid.