1. A method for changing a software in the memory of an electronic control unit, wherein the memory includes at least one read-only memory with multiple original program routines stored therein that constitute at least a portion of the software and that are processed by at least one processor of the control unit, the memory has at least one working memory for storing volatile data, and the memory also has an overlay memory, the method comprising: assigning each memory address from the overlay memory to a memory address in the read-only memory by an assignment information item, with the result that, when an overlay functionality is activated, instead of processing a program routine instruction at an address in the read-only memory, the processor processes the instruction at an assigned address of the overlay memory; storing, during a run time of the control unit, at least a functional part of a bypass routine that is to at least partially replace an original program routine in an address range in the overlay memory, or storing a jump instruction in the overlay memory as the first part of a bypass routine that refers directly or indirectly to a second part of the bypass routine that is stored in an address range accessible to the processor; and creating an assignment information item, in order to activate an overlay functionality, that assigns the address and/or the address range of the overlay memory to an address or address range of the program routine to be replaced or the portion of the program routine to be replaced and this assignment information item is activated or deactivated through an interface of the control unit, wherein an activation of the overlay functionality takes place at a time when machine code to be replaced by the activation of the overlay functionality is not being executed by the processor of the control unit.