If a process happens to update other process' outputs, then we would consider it as project document updates.
Per PMBOK4, Define activities generates activity list,attributes and milestones, by taking inputs from scope baseline, PMIS (EEF), scheduling methodology (OPA) etc. The process does not update any existing project documents by that time.
For example, Risk response planning may identify changes to WBS and the activities list. This is another PMIism, may be!
I feel while decomposing the work packages into activities, you might realize that few work packages were missing, and you might as well change WBS. If PMP is not yet approved, thats okay, other wise you would go through change control process to reissue scope baseline.
Just wanted to share my viewpoint!
Regards,
Chary