Add a new Delivery date control method call something like "Net requirements based ATP" or "First come first served ATP".
The current ATP calculation looks at when the requested inventory for a new demand is "available" at the soonest. 
This new delivery date control method will not just look at the soonest date, but will consider other demand already in the system all the way out to the ATP Time fence. If the newly added demand will cause any  of the existing orders to not be able to be fulfilled as currently scheduled, then the new demand will get a ship date out at the ATP time fence.

In a complex business with demand from Sales order, Production orders, and transfer orders all at the same time and some of them with dates that move, placing, and then maintaining, reservations is a very tedious job.  This maintenance is necessary to make the current ATP method work.
If, or should I say WHEN :)  this new Delivery date control method is available, users won't have to worry about reservations.  Their current order ship dates should all be valid regardless of whether it has a reservation or not.  Newly entered demand will not "steal" inventory that a soon to deliver, but not yet reserved demand, needs and is counting on.

This method would compare the quantity of the new demand to the Net requirement's Accumulated value.  If the new demand takes the accumulated value negative any time from today to the ATP time fence, then the new demand would get a ship date out at the ATP time fence.