Changing Sales order quantities will affect the inventory status, shipment qty and invoice qty after confirmation. 

There 2 golden rules to changing qty: quantity on a sales order can't be lowered below the amount shipped and the amount drop shipped. That will be the limiting factor. You may cancel a dropship so you can reduce the qty further on a sales order.

If there are multiple invoices or a partial invoice for a sales order, the qty changed on the sales order will not reflect on the invoice. The change will have to be done manually.

If qty has been picked and packed and changes are made to the order qty, the system will update the changes to the pick and pack qty as necessary. For example, if Order Qty is reduced from 5  👉2 and all 5 Qty are packed, the system will reduce the packed qty to 2. 

As the qty is changed on the sales order, the allocation, available to pick and committed qty for the order will get updated accordingly.