Method of exchanging a printing unit at a running rotary printing press

A method of exchanging printing units at a running rotary printing press, wherein a first printing unit is withdrawn from a printing medium, and a second printing unit for printing an identical image with an identical register is set against the printing medium, comprising the steps of:

• applying to a printing cylinder of the second printing unit, prior to a start of a printing operation, a register printing block that is elevated relative to a regular printing block of said printing cylinder,
• prior to a changeover of the printing units, setting the second printing unit against the printing medium to such an extent that register marks are printed with the register printed block, without an image being printing with the regular printing block thereof,
• detecting the register marks and adjusting the register of the second printing unit by reference to the detected register mark,
• when the register has been adjusted, exchanging the printing units by withdrawing the first printing unit and fully setting the second printing unit.