Imaam moves while reciting the Takbir

As the Imaam, when is the correct moment to recite the Takbir; do I recite the moment I start moving from one position to another? Or is there an acceptable period within which to pronounce the Takbir while I am moving? And will my Salaah be termed markooh/nullified if I pronounce Takbir but I am already in the next position?