Hi Rob,
I am not from Laminar Research; just a flight simmer from downunder.
Firstly X-Plane 10 and X-Plane 11 are totally different products both developed by Laminar Research. Just as different as a Volvo and a Toyota car. You will have to purchase the new version ie X-Plane 11. At the time XP11 was about to be released there was a sweetheart deal whereby you purchased XP10 and then you automatically upgraded to XP11. That arrangement closed down within an extremely short time after XP11 was officially released.
Because of the difference in each version you may need to upgrade your PC. The new system requirements can be found at the following link at https://www.x-plane.com/kb/x-plane-11-system-requirements/ The serious flight simmers build a system equal to or of higher specifications than the Recommended system. If you need to upgrade go for an Intel CPU and NVidia GPUs for best performance. Because XP11 generates heat you will also need good/adequate cooling in your PC case. Although X-Plane is developed using a Mac system, Laminar Research staff have been known to advise in the purchase of a windows based PC system. By going this way you can upgrade individual components far easier than using a "sealed" PC system.
I strongly suggest you purchase directly from Laminar Research and support their development work. Although it may be slightly cheaper, don't even contemplate the Steam version. Steam is a 3rd party hybrid taking a commercial advantage of the development work by Laminar Research. Steam updates are always behind Laminar Research in any update release. Steam is not part of Laminar Research.
I also understand, and seen in this forum, whereby there is another company selling X-Plane at a cheap rate. It appears that many are being "burnt" by this reseller with "dodgy" password/keys that lock your purchase after a couple of days. Refunds?
Good luck
Glenn