Is going from 415 cc high profile to 600 cc high profile too much?

Dear Briibarb.   You posed excellent questions, however , without front and side view pictures and your chest and breasts circumference measurement in inches, which are critical for the correct implants size selection, it is impossible to give you appropriate answers.   It is in your best interest to consult with experienced board certified plastic surgeons who operate in accredited surgery center for your safety. Most importantly, check the before and after pictures of redo breasts augmentation surgery in the photo gallery, to make sure that they are numerous, consistent and attractive with nice cleavage, perky, symmetrical and natural looking.                 Best of luck,                                Dr Widder

To start, an implant size of 600 cc is certainly larger than average, but does meet some patients needs.  Generally speaking, I discourage patients from starting a cycle of progressively larger implants in search of an elusive fullness.  The larger implants, and 600 is one, can lead to an even faster drop simply due to the weight of the implant.  

Unfortunately there are no pictures. It should not be a problem but I would make the following changes. It’s important that the diameter of the implant fits the width of your breast. Mentor and Allergan do not make a 415 cc high profile silicone implant. For arguments sake let’s assume you have a 425 cc high profile silicone implant. This has a diameter of 12.5 cm and a projection (aka thickness) of 5.0 cm. A 595cc Mentor Xtra is 13.1 cm in diameter and with a projection of 6.3 cm. If this would be to wide for your breast, then I would recommend a mentor 590 cc ultra high profile implant. This has a similar diameter of 12.5 cm and a projection of 6.3 cm. If your breast can accommodate the wider mentor xtra then that would be an option. You would need a physical exam to determine what option is best for you. I hope you find this information useful. For more information please watch the enclosed video. 

Yes, it is likely that you will be able to safely utilize (and will see a significant difference with)  larger implants with about 200 cc more volume.   
***The much more important question with be: which breast implant (volume and profile) will be the correct choice when it comes to  achieving your desired outcome when it comes to breast size/shape/symmetry…?  Online consultants will not be of much specific help to you in this regard.
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. I have found that the use of words such as  “natural” or "D or DD cup” or "much difference" or "too fake" 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 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).  Viewing the patient's chest wall in the upright and supine positions, with temporary sizes in place, help select  the best breast implant size/profile for the specific patient.
 Best wishes. 

Please note that without any photos, it is impossible to answer your questions to any specific degree. You mentioned Kate Upton - she has large, natural breasts and has very little upper pole fullness. She is in fact a great example of the "non-implant" breast. If you are expecting breasts like hers with a 600cc implant, you are likely to be disappointed. Rather, you state that your breast skin has stretched and you don't feel "big" any longer. You may simply need implant repositioning and a supportive breast lift to improve shape. For women with implants that have dropped over the years, you can make the breasts "high and tight" with an implant exchange to a higher profile and an "internal bra" mastopexy for long term support. This can create a much fuller breast that sits higher on your chest and gives the impression of larger breasts without increasing implant size. Hope this isn't too confusing!