One possible approach is the following. Create two categories one for the undergrads and the other for the grads. Put all the shared grade items (forums, mid-term, and final exam) in the undergrads category. In the grads category add 3 gradebook grade items, one for the forums, another for the mid-term and a third for the final exam and set the formula in each of these 3 grade items to take the grade of the respective original grade item (e.g. =[ [forums] ]). Add the grads' paper grade item to the grads' category. Then set the aggregation method of the grads and undergrads categories to weighted mean and set the desired weights of the items in each category. Now you need to exclude the undergrads category from the grads' course grade and the grads category from the undergrads course grade. For that you need to edit the grader report and for each student edit the category total of the opposite category and tick the excluded checkbox (I hope you don't have too many students; but then again you do it only once).
Hope this works for you.