I came across this issue recently after my company merged with a different one and we all received new emails. As we were working on starting anew, the question continued to come up:
How can I merge my current Google Drive with a new one?
Turns out... you can't. But there is a simple work-around that pretty quick and easy.
Take all of your existing files and put them into one folder. Then, share that folder with your new Google Drive account and bingo, now you can access all of your old Google Drive documents through your new account.
Admittedly, you'll still have to give yourself editing access to the documents (when necessary), but if you have email forwarding set up, it should be pretty painless to grant yourself access upon opening the document.
Hope this helps!