In my experience, preparing both versions of the course would be about a 25% increase in workload, and I was the instructor that helped other instructors figure out how to get things done in online course management software platforms, I imagine the workload would be far greater for the less tech savvy. A common experience for adjuncts, who do a significant proportion of the instruction at a college, is to have a contract that pays for about 70-80% of the actual hours that you end up having to work to effectively teach a course. The only way to get through a semester with a shred of sanity is to be very well prepared; my prep work would usually start at about 5-10 hours a week at the beginning of July (unpaid) for classes starting after Labor Day, with about a 20 hour workweek the week before classes started (maybe paid 5 hours for showing up to school and department meetings and training seminars).
Having certainty about what the semester is going to look like will make the online course delivery far, far better. I have a dozen or so friends who are University Profs, and they're all in favor of defaulting to teaching this fall online (excepting laboratory and art studio types of courses) given the extremely high odds that shutting down or greatly reducing in person instruction is going to happen.
