I want to downsize from 400c implants to 300cc/325cc. Will this be a big enough change for me to go do surgery again? (Photo)

swap out current implants for implants 75-100cc smaller, may have to change profile/style of implant at same time FYI

Given your concerns/goals, you seem to have a reasonable plan. Keep in mind as you are thinking about your goals that sometimes adjustment of the breast implant pockets (capsule adjustments) may be necessary to achieve the outcome you are looking for.  Much will be dependent on your physical examination;  specifically, factors such as the amount of redundant skin and  quality of skin elasticity will be important.
 The 100 cc you are considering downsizing will likely make a "noticeable difference"; what cannot be determined online is whether it will be the exact difference necessary to achieve an outcome that you will be pleased with.
My best advice: more communication.Generally speaking, the best online advice I can give to ladies who are considering revisionary (downsizing) breast augmentation surgery is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or "down a cup size” or "full B, small C cup" etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. 
3. Once you feel you have communicated your goals clearly, allow your chosen plastic surgeon to work with you in determining the best plan to achieve your goals.
I hope this helps. Best wishes.  

Thank you for your question and providing photos. It appears that you have a very nice result and your breasts do not look to big for your frame. I understand you want to be smaller. This can easily be done without significant down time. You should see a decent reduction going to 300cc's. I always recommend you go at least 100cc's smaller to see a difference. This will most likely require changing the profile of your implant as the width is very important. Best of luck