Theres some good advice here on this forum and from the other discussion threads from people that have been there done that.
Essentially you need to figure out whether she wants to get bigger because of you. If not then why is getting bigger so much more important to her now? Did the size up to DD excitied her? If so in what way? I mean she could be under the notion of going through the excitment that puberty brought to her again, when they grew naturally.
I guess I am trying to say that their are 1001 reason s to get the boobs bigger, and 1001 reasons not to, the important aspects about the whole process is that what is motivating it, does the benefits out weigh the burdens, and of course after all that, is it worth it for all involved both physically and emotionally. Only then will you know that this notion of getting E-cups is either a fleeting wilm or a true ambition. At the end of the day what is going to be happier for both of you's, bigger boobs, or realising that there is something deeper driving the need for them, that needs to be taken care of first.
I think you get all the information about this process, and you can get personal testomonials from some people in here, but it's your lives, we don't know how they work, only you do, so therefore it's all up to you and your girlfriend.
We can guide and advice, but we shouldn't and realistically we couldn't help you decide for you.
Catch ya
DD_Redshoes