A method and apparatus for reallocating in the local memory of an implantable medical device for upgrading operating software stored in the Code portion of the local memory. The local memory is managed so that new software e.g., a code segment, is accepted into memory provisionally, and then validated before being used to replace existing (i.e., old) software. Each new segment is received in its entirety and its integrity validated before the old software is overwritten. Data log information is downloaded to an external programmer at the start of a software upgrade operation to temporarily free memory space without losing any information. This freed memory space is then reassigned for the purpose of temporarily storing new code segments until they can be received in their entirety and validated. A software copying component is transferred from a local memory code portion to a data portion to allow new code segments to be written into any area of said code portion.