Hi Karan,
As you said, due to some business requirement you are changing the RDD of the Sales document in ECC using customized solution, the calculated RDD would be taken forward for ATP check. APO can understand only the RDD that is being passed to GATP system for Availability Checking purpose. So, if the business is OK to execute all the ATP processes as of the date that was determined by your custom solution, you should be good. Of course, if you're changing the dates to future or past relative to the actual RDD asked by the customer, there could be changes in the confirmation. Also, in the post you mentioned that "Can I create delivery for the changed RDD" ? Please note that deliveries are always created against the scheduled MBDAT. Because, there is a lot more processes involved between MBDAT and Final Delivery date in terms of picking, loading and transportation.
In your example, if there are 1000+ orders for 15th day and if there is a stock/receipt for 100PCs, and if you change the RDD for one of these 1000+ orders, system tries to do the availability check and the MBDAT falls in line with the stock/receipt element then, you would get the confirmation on 05th day. However, if you run BOP; system decides the confirmation based on the priorities. I request you test such scenarios in the system and present it to your customer to ensure that they are in line with what you're doing or proposing in terms of solutioning.
Hope this helps.
Babu Kilari