Bike Fit for 5'3" (1.60m) woman, 47 or 51 cm?
The bike shop is correct. This will depend on what model of Cervelo you have but looking at the Classic Road geometry here, 

48cm: stack 505mm, reach 560mm

52cm: stack 530mm, reach 569mm

That's annoying but kind-of does make sense. At the small size extreme of frame geometry it's hard to reduce the length of the bike and leave enough space between the wheels, because the wheels are the same diameter as larger frame sizes. This is why some small frame road bikes are built around 584mm (650b) wheels instead of 622mm (700c) wheels.

It's possible that 0.9cm reduction in reach is enough, and you can raise the bars up a couple of cm with a different stem.

If you are thinking about buying a whole new frame and transferring wheels and components over, you should probably think about a whole new bike of a different model/brand that fits you better. If you sell your current Cervelo the overall cost may be comparable to buying a frame.