For this answer I am assuming you have a utility patent that already issued, not that is currently in prosecution, in which case the answer will likely change a bit. Whether you need to file anything with the USPTO really depends on what the changes were. If the changes were made because the original patented invention did not operate at all, then you would need to disclose the fact that the disclosed invention did not work to the USPTO. That doesn't sound like your issue though, just that you made a few small changes to possibly improve the design. In the case that the original design worked you would not have to disclose the changes to the USPTO.
If you want to protect the small changes, and the changes were not claimed/protected by the original patent, there are two alternatives available based on the amount of disclosure of your original patent (something that must be determined by visiting a patent attorney/agent). In the case that the changes you made were not disclosed in any part of the original patent you would need to file a separate patent application to gain protection for the small changes, if you desire. If the changes were disclosed and not claimed in the original utility patent, you can add protection for the small changes by filing for a reissue of the patent within the first 2 years after the patent grant; if it is past the 2 year period you would need to file a new patent application. Visiting a patent attorney/agent will help you to determine which action you are able to take. Note that even if your design has changed a touch whatever was claimed in the original invention will still be protected, so if this is still embodied in your new design, that part of the new design will be protected by the original utility patent.
Finally, if you can use the same patent number on the new design depends on what is claimed in the original utility patent. If the invention claimed in the original utility patent is still embodied in the new design (even with the small changes), you would be able to continue using the same patent number on the new design. If the new design does not embody the invention claimed in the original patent, you would be required to stop using the same patent number on the new design. To figure this out, I would strongly suggest taking the original utility patent into a patent attorney/agent to review and presenting the attorney/agent with your new design to see if you would still be able to use the same patent number on it.
Answering of your question is merely general advice and does not constitute legal advice. None of the statements or implications made by this answer creates an attorney-client relationship with the attorney answering the question. The statements made in this answer are not to be solely relied upon and you should meet with a competent attorney to discuss any concerns you may have regarding this answer.
You don't have to file anything with the USPTO.
Product designs are always changing, and products actually being sold usually look quite a bit different than the drawings in the patent (which likely was filed years earlier).
You can use the same patent number as long as the "claims" in the patent still cover your revised product. A quick read by a patent lawyer will answer that question for you.
(949) 390-2717 - Of course there's more to it! Plus, we don't have an attorney-client relationship. This brief comment is for information only, and must not be relied upon as legal advice.
Short Answer: You can't add "new matter" , meaning, change the invention, without changing the priority date (i.e. the date the patent office will assign to your claims.) It is not clear if your "small design change" will require modifying your existing utility application. That determination should be made by a patent attorney or agent. If the change is significant enough, then you may want to file a Continuation In Part (CIP) application, which is a new application that is based on the old one with some new matters added. Claims based on existing material will receive your original filing date, while claims based on the added material would receive the date of the new filing.
If the change can find support by what is already in the application, then you can perhaps add new claims relating to the change without filing a new application.
You should consult with an attorney to determine what is the best approach for you.