My first reaction is: why are you teaching a grammar point about which you are unsure when your job depends on it? Is there any way you can change the grammar point you're teaching? I suggest you teach something you know about.
An upper-intermediate class would be able to deal with modals for deduction in the present and the past; for many languages, this concept translates directly. If you have to teach that, I don't think it will be difficult if the class really is upper-intermediate.
I suggest you find out more and come back here if you want more info.
All the best,