The difference between L-1A and L-1B is based on whether you are in management or not. Since you came on an L-1B I assume you were not a manager in your firm's overseas office before being relocated to the US. As far as I know (and I may be wrong), you cannot covert to an L-1A unless you go back overseas, serve as a manager for the required duration and then return on an L-1A visa.