I have a provisional patent. What if I am talking to a company and they need to change the design for manufacturing purposes? I

As an inventor, when you contract with an engineering firm to implement an embodiment of your invention, you should always incorporate an "invention rights" clause in your contract that provides for the assignment of any improvements to your invention to you are your company--particularly where those improvements are those of the engineering firm. That is the "price" the engineering firm "pays" for your business. In this way, you will be able to interact freely with your engineering firm without viewing the relationship as adversarial. That is critical for bringing a product to market cost-effectively and in a timely manner.

If you do not have such a contractual provision with your engineering firm, you should do an "addendum" to your contract to establish these rights. You may have to "pay" the firm some money as "consideration" for the addendum...perhaps even a small amount, but it is critical to do so.