I also studied computer science (in Europe) and eventually became interested in economics and financial markets. I ended up combining both and writing my graduation thesis at my schools Econ Dept about different automatic trading models.
I would advice you to figure out what part of economics it is that interests you the most, and whether you want to combine it with anything you did before or simply start all over.
If I were you, I'd also start reading up on some relevant papers in the field and look for the good stuff in their bibliography. Don't be too shy to mail the authors and ask for possible follow-up on/advanced related material advice - most of them are friendly enough to share that kind of knowledge with you (especially if you praise their work and promise quotations in any future publication of yours
). Once you did that, you should have acquired a broad horizon about the research hubs in your particular field of interest and should then be able to choose your school accordingly.