Please go to your Amazon key settings and click the Disconnect Button.

Go to Settings --> Marketplaces --> Amazon and click the country you're selling on.

You will see a link there that will provide you with detailed instructions to authorize us as a developer within your seller central account.  Click the Disconnect button to get rid of the old keys and go to the new format.

Click the detailed instructions link to access the instructions on authorizing the app in your seller central account.

Once you click the Disconnect button, you will get a pop up that says Are you sure you want to disconnect Amazon from Sku Grid.  Click OK.

You will then get another pop up that says Amazon keys will be lost.  Are you sure? Click OK.

You'll then see a 3rd pop up that says Amazon keys deleted.  Click OK.

Once that is done, click the Connect button and you will see that the key fields have changed.  Access Key and Secret Key are no longer required.

Simply take the keys that you have from your seller central account and the MWS token that you've generated in Seller Central and enter them into the corresponding fields in Sku Grid and save settings.