Hello,
Could someone tell me how many changes do I have to make to convert the project, so I can use it with vive VR.
Thank you.
too many to be worth it
You need to make new game out of it. Movement, shooting, almost everything related to player must be changed. And on top of that you probably get most people dizzy and puke while playing in VR. I recently tried Detached, its very slow and probably as VR friendly as you can get for FPS, but still cannot play it more than 2-3 minutes.
FPS for VR is very bad idea, first you have small market of people who have VR, then you have it even smaller, limited to ones who have VR and do not puke from movement. There is reason all VR games use teleporting.
I’ve had similar experiences when trying to convert an existing project to VR. It’s not too much effort to get it working, but unless it’s designed for VR you are going to run into a lot of issues. People will get sick because traditional FPS games move a bit faster, you are often looking, sidestepping, etc. Interface is often a problem. So while you can rig something up so it runs in VR, its unlikely to be a good experience for end users unless it was designed with VR in mind.
The sales of Onward would with disagree with you. 
I’m actually developing an archViz project for PC, so the only thing that I would need to change is the player controls and the menus.
Thank you for elaborating.
If that’s the only thing you’re after, then import default VR template that came with 4.13. I works out of the box when it comes to movement (teleporting). As for the menus, that is more complicated than simple UI widget, but 4.13 will definitely let you set up all VR menus.