If I understand you correctly, given a the only way to change a constant Total Rise distance to change from a 7" rise to an 8" rise you total distance must be a multiple of both 7 and 8. Assuming it is 56" of total rise than change the number of rises to 8 for a 7" per tread rise, or 7 for a *" per tread rise.
If the climb of the Total vertical Rise of the stairs is a variable than just increase the Total Rise.
I am sure you know all of this. It may be that you just don't know how to get to the Dialog interface in order to make these changes.