I grew up in the northwest suburbs of Chicago (which has a few rolling hills) and spent a few years in Miami which is even flatter than Chicagoland. You should be welcoming the terrain of SF instead of being concerned about it. My cousin who used to live in SF is in Gainesville right now, and she isn't mclovin the flat humid rides in the area.

As others have mentioned, you can avoid the steepest streets in SF and get a low-geared bike with good brakes if you plan on riding any of them. If you've been regularly commuting and are in decent shape, you should be fine. Riding is also going to be a lot more pleasant year round because of the weather. Say goodbye to the humid summers and the cold winters that make Chicago such a wonderful place to live.