Would you please inform us when these brochure changes will be ready? If you use "could", it is asking if they are able to give the information. Using "would" is requesting them to give the information.

Try this: "Please inform us when the brochure changes are ready." Or if you are asking for a time when the changes will be ready you can ask: "Could you please inform us when the brochure changes will be ready?"

Hi, I was not too sure what it was suppose to be but I changed it to the sentence below, I hope it helps. Please could you inform us when the changes in the brochure are complete.