Substitution of DATEV (German Topic)

We are looking for some information about substitute an existing DATEV-environment with MBS. Our company is an inhouse use of DATEV, that means our accounting-department work like an tax consultant-office. The tax consultant act as a supervisor. Our plan is to substitute the DATEV-installation with Navision. This shall be realized in steps. Until we transfer all functions to the MBS-System we want to use the whole functionality of the DATEV-system. At the End there should be two Systems parallel. Then we can decide to switch of the DATEV at any time we want. Is there anybody with experiences in that area?