Abstract: A method and apparatus for performing an on-line replacement of an industrial process control processor module via loop-by-loop transfer of control blocks is described. After replacing a backup control processor module in a redundant processor configuration with a first upgrade control processor module, the first upgrade control processor module is placed in a partially operable mode enabling reading of control values from a control data storage. However, the partially operable upgrade control processor module cannot write values to the control data storage. Thereafter, control loops for an industrial control process are transferred, one loop at a time, from a primary control processor to the first upgrade control processor. After each loop is transferred, the capabilities of the first upgrade control processor are expanded to include write access to the control data storage associated with the transferred control loop.