Definently possible to transition over from ACN to an Investment Bank. Some areas are easier than others. If you're objective is to move into middle/back office or into an operations role as a Project Manager, then there shouldn't be any real issues, so long as your exposure to the technologies/products/business areas is relevant. I would say, if you're thinking about the move, make it soon, since the longer you leave it, the harder it will be to make the move, since the more experienced you are, the more relevant and specific your past experience will be to the role.
If on the other hand you want to make a move into front office, it will be very difficult to make a direct move. If however, you are determined to do front office, then I suggest an MBA from a Top 10 business school, with a solid case for wanting to make that move. If this is done properly, then its certainly possible. I left ACN in 2000, as a C2 and did my MBA at Columbia and have been working in Front Office as a VP within Equity Derivs for the past three years. If you want any advice etc. drop me a line and I'd be happy to help.
Best of luck