Exchanging 350cc mod profile implant for 425cc or 450cc Silicone HP implant. Too Much?

Thank you for the question. Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.
Generally speaking, the best online advice I can give to ladies who are considering revisionary breast augmentation surgery ( regarding breast implant size/profile selection) 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. For example, I have found that the use of words such as “natural” or "D cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
In your case, my best advice: more communication. It will definitely worth spending more time (prior to the date of surgery) with your plastic surgeon to discuss further.  As you know, it will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.
I hope this (and the attached link/video, dedicated to revisionary breast augmentation surgery sizing concerns) helps. Best wishes. 

In order to size you up, you would have to be seen in person.  A 25 cc difference is not a lot if that is the size you have chosen with your surgeon.

Thanks for the great question. In my opinion, if you're going to exchange implants for larger ones then there needs to be a noticeable difference in the size. Depending on what the base width of your breast is, there's a limit to the size of the high profile implant what will be able to be placed without having the implant fall into your armpit. Also, the difference between a 425 and 450 is almost nothing and you likely would not even be able to tell the difference between the two. A recent study showed that each increase of 150cc took women up one cup size. So for you going to a 450cc would put you somewhere around a full D to small DD. As for your base width, I don't think the narrower base width of the HP implant will make a huge difference in the space between your breasts. Best of luck on your surgery!