Unfortunately, in most cases where a transition occurs between contract developers prior to a shipped product, it's almost *always* the case that the new developer starts the code largely from scratch. When problems dictate a required transition to a new developer, it usually means that the previous developer left enough of a mess (in the codebase) that no good developer would want to try to waste your money and their time cleaning up the mess.

Happy to talk with you about your current predicament and offer you advice based on more information.