Will 200cc make a big difference? I currently have 420cc HP gummies. (Photo)

Yes, it is likely that you will see a significant difference with about 200 cc increase in breast implant volume.   The much more important question with be: will the 620 cc breast implants be the correct choice when it comes to  achieving your desired outcome?   
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 "big difference" 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. 

hello flowerbenz  It’s important to know what size bra you currently wear. Is it a 32,
34, 36 bra, and, of course, the cup size? The strap size is important to
know because that affects the diameter of the cup and subsequently the
diameter of the breast implant that we choose. We also need to know your
height, your weight, and it’s helpful to know your dress size as well.
In addition, it would be very helpful if you include photos of
yourself. A front view, a side view, and a three-quarter view are most
helpful. This will let us see if there are any breast asymmetries, if
the breasts are narrow, if the breasts are wide, what your chest wall
looks.
Breast Implant Sizing (generalization for most patients)
32A to 32C needs about 250-275cc34A to 34C needs about 300-350cc36A to 36C needs about 350-400cc

200 cc increase in implant size will make a difference, but whether or not that will be big enough to meet your goals is unknown. It depends on what you consider a big difference. In the average sized patient, adding 200 cc would increase your size by approximately one cup size.