Adding QACA to an existing safe harbor match plan

An employer wants to amend their plan to adopt QACA.  Can they have QACA apply to just employees hired after a certain date or does QACA have to apply to everyone? They currently have a basic safe harbor match plan.   The QACA match differs from the SHM and having 2 different SHM formulas in the plan is not permitted (I think).  

The employer would prefer not to have existing employees have to go through any enrollment process or have anyone currently employed defaulted into the plan.  But if everyone currently employed has an affirmative election to participate or not in the plan, would those employees really need to complete anything for the QACA?  I would think not.

Thank you for replies