Update Custom field (lookup table), PS 2007

Hi, I have some custom workflows for Project Initiation and Project Planning. Project Initiation creates a Project plan and publishes it with Project Status (custom filed) default value "A". After PI WF finished we need to start the PP WF manually. At this point we need to update the Project Status Custom field to "B" from A. How can we update a custom field value (lookup table) using PSI? Or is there any other way to do that?

Custom field: Project status with lookup table values "A, B, C and D" (all at level 1) with "A" as default value.