Not quite sure what you are asking in your question. Completion Date moves the same amount as Planned Completion moved due to the compensation event. If planned Completion was two weeks earlier than Completion Date on the accepted programme, the Contractor has two weeks terminal float which the Contractor owns. If the CE moves planned Completion by one week, once the CE is implemented it will move Completion Date by one week.
It is just a question of timing as to what milestone moves when depending on when the programme is submitted and when the CE is implemented (agreed). However - that is the precise reason the contract has these two distinctly different milestones. I always say planned Completion is “reality”, with Completion date indicating “liability”.