The Tesla website doesn't make any statement that the standard range pack can be unlocked in the future (EAP shows a price for purchase at or after delivery). If Tesla offered the ability to unlock the battery pack, they probably wouldn't sell any of the extended range models...
In practice, you'll normally charge to 90% of actual pack capacity, and charge above 90% only if you're planning to start a long drive, and you need the extra charge to reach your destination or first charging stop. With increasing coverage of the supercharger network, this is needed less and less.
With the standard battery pack, you'll likely be able to charge to 100% of the standard pack capacity, because you'll still have the extra 8-10% of pack that you aren't able to use. So unless you plan to make many long drives during a year, you probably won't miss the extra capacity of the unlocked pack.
And even though we purchased an S 100D and X 100D in the last 2 years, it might be tempting to save some $$$ and go with the standard pack the next time.