Contrary to e.g. the aid climbing difficulty scale, sports climbing and bouldering scales are comparative (at least in the grades that came up after the seventies). So a grade does not directly tell how hard a climb is, it just says how hard it is related to others. You can see this very well by sometimes drastically different hard routes with the same grade in different crags or gyms. There are two gyms within 10 driving minutes of each other, where equally hard routes have like two French grades difference. What I want to say with this: There is no definite way and you should not concentrate too much on grades.
Therefore the only way to transform the grades is by finding a "normal" overhanging moonboard and compare it. I will give some ideas of what makes it harder with an overhang, but these factors cannot be quantified as in 10deg is half a grade.
In terms of holds it is pretty easy to see (isolating it from other factors): Big jugs and slopers are not much different, crimps are. The difficulty of holding a crimp is more or less given by its size and "positivity" (angle of contact area against direction of gravity). This obviously directly changes with the angle of the overhanging wall. One might even come up with some quantification in terms of grades for this.
Then there is weight support by legs. Ideally in climbing you support all your weight with the legs and the hands/arms are only for keeping you at the wall. Of course this is no longer the case for harder routes, but it still holds that you should support as much weight as possible with your legs. In an overhang you have to lean back, so even with perfect foot placement (kneebars excluded) you will have to hold significantly much more weight with your hands. This is actually just trigonometry, so an estimate of how much more force could be made and translated to a grade, but again thats useless (see above).
The third thing I come up with right now is body positioning. In an overhang you need to kind of fixate yourself with opposite arms and legs between two holds. This needs a whole lot of body tension. In addition it is a very different way to hold and stand on a hold than when you are climbing a vertical face. The direction of pull/push, the way you grab it may be completely different. So a routesetter will always have this in mind: If a route is overhanging he will do it completely different than on a face. Therefore your routes will be suboptimal as they were designed for an overhang and you will use them vertically. This can mean they are disproportionally easy, but it could also mean that it is just as hard as in a overhang.
I suggest you either mount it as it was designed for, or you set your own routes for vertical walls. Setting routes is very difficult at first, but you learn a lot as you have to think about movements in advance, which is an advanced skill, but a very useful one.