I put in my deposit in Oct 2015, confirmed end of January this year - for an X 90D, was told to expect delivery mid-year. Then I saw a new house, and decided to sell my house and buy the new one the beginning of March - put down my deposit, got approved for the loan, etc. Then all of a sudden I get notice my car is going to be delivered by March 31! Got a VIN # and everything.
In the middle of buying a house with the old one not sold yet is NOT the time to do 100K in car financing - so I told them I needed to delay. They were great, I asked them to call back in April for delivery any time after 5/15 (closing for the house was the first week in May). They did, had me reconfirm, and then I got a new VIN assigned.
But if you want to delay AFTER confirming, I don't think they'll do that - once you confirm your order, they try to get started on building. My situation was a little unique - they ended up selling the first car to someone else, I guess. But if I'd agreed to a date, then wanted to wait for new features I don't think they'd have done it.
If you really want the new features, just wait to order - I suspect in the next few months the wait times will be under 60 days for any new S or X order.
But there will always be "something" coming - the X is the most amazing car I've ever owned, even if there are new features later, I don't believe I'll ever regret getting it NOW.