I am a 49 year old transsexual and got 400cc implants one year ago, can I go bigger to maybe 600cc? (Photo)

It is possible to #increase your implant size with a #revision. However, the #pocket will need adjustment and a #breast lift may be necessary; to better #support the new implant. Additionally, a mesh for further support may be helpful depending on the #implant size and skin #elasticity. You will learn if you are a candidate to increase implant size. and more about the #options best suitable for you, during a one-on-one in-person #consultation.

It is likely you could go larger, it would depend on your tissues and anatomy. Be sure to see a board certified plastic surgeon who is experienced in gender affirming procedures as there are differences in surgical technique to get the optimal results.

Thank you for the question.  Generally speaking: yes, It is possible to increase breast size further after initial breast augmentation surgery.  Careful selection of breast implant type, size, profile (sometimes along with adjustment of breast implant capsules) will be necessary.   
You will find that online consultants will not be able to provide you with specific advice in regards to selection of specific breast implant size/profile. Ultimately, careful physical examination, measurements, and communication of your goals 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. I have found that the use of words such as “natural” or "C or D cup” or "up a cup size" 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.
I hope this, and the attached link (dedicated to revisionary breast augmentation surgery concerns), helps. Best wishes.