I am not with Laminar Research either; just a flight simmer who does not support the steam version of X-Plane.
So have you purchased the original and official version of X-Plane from the developers being Laminar Research (LR). I have had some difficulty trying to verify the site you mentioned you have purchased from. Is it selling official versions of X-Plane?
Why do you want to change to steam? The steam version of X-Plane is a 3rd party hybrid taking a commercial advantage of all of the development work performed by Laminar Research in developing the top flight simulation package in the world.
Steam will not mix with the LR version. They are different packages. In the development work by LR they allow users to download progressive updates while developing a new version so that knowledgeable x-plane users can find problems while creating the new update. Steam offers no progressive updates only what becomes available when LR makes an official release.
There is no connection between LR and Steam. It is a totally independent business.
The comment by HighFlyer69 in responding to your question strongly suggests to me that steam is highly protective of their hybrid product by not allowing other outlets to market their product. On the other side of the coin LR has a number of commercial outlets whereby you can purchase disc versions of x-plane from retailers such as Aerosoft who are licenced to sell in Europe and the South Pacific region on behalf of LR.
If you want X-Plane to develop further then "dump" the idea of having a 3rd party hybrid steam version and totally support the developers in all of their work.
So, I go back to my question of why do you want to convert to steam?