Hi windexter,
I am a flight simmer like you and have no connection with Laminar Research (LR).
The latest X-Plane 10 version is 10.51 and I understand there will be no more upgrades to XP10 before and once XP11 is released.
When LR progressively upgraded/provided a new XP10 version through to 10.51 there was no ability to go backwards. his has been the situation for some time. My original purchase was XP10.25. The "no backward step" is also spelt out in other Q&A questions within this forum. Sorry, can't provide the reference/link. Your current and only choice is to retain what you have or upgrade to version 10.51.
As a sideline issue, XP11 will not over-ride XP10 when loaded onto your PC. It is a completely different concept/development. You will have two versions allowing you to switch between XP10 and XP11 when you so desire.
In other Q&A questions in this forum the addons and or plugins you have for XP10 will work for XP11.
Hope this helps.
Glenn