Since X-Plane 11 is an entirely new version of the software, the purchases made to get X-Plane 10 will not transfer to it. However, X-Plane 10 will continue to function and should still provide you years of enjoyment if you do not wish to buy X-Plane 11.
We believe very best bet here for our users is to buy each version early on during its development cycle, which gives you the longest number of months of enjoyment and the greatest number of free updates before the next major Version (which you would also be required to purchase) is made available.