is going bigger a mistake? 380-550 or 600

In regards to breast implant size/profile that will best help achieve your goals, I think it will be in your best interest to spend more time with your plastic surgeon. Careful measurements, dimensional planning, and careful communication will be key.
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're 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 which operation and/or 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 "C or D cup" or "too big" 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.
The use of computer imaging may also be helpful during this communication process.
Regardless of what you might hear, there is no reliable correlation between volume of breast implant used and increase in breast size achieved.
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.
I hope this, and the attached link, helps.

The Tuba technique is challenging and very hard to control the pocket.  Your new surgeon understands that.  Please consult with your surgeon and you can get more information on the Smart Beauty Guide!

and put over your existing breast would give you a good idea of what size you will be.  Is your frame big enough to accommodate that size without looking fat?  Try sizers at your doctor's office with an unlined bra to help you see what options you have.  As for the TUBA, its a challenging procedure and very few surgeons employ it...