How likely is it that a simple ovarian changes into a complex cyst?

Not likely: "A simple ovarian cyst and a complex ovarian cyst" are sonographic diagnoses based on some criteria to describe what is visualized on a scan. Simple cysts are usually considered transient and benign structures that tend to resolve spontaneously. Many/most complex cysts require further investigation to rule out or confirm an ovarian malignancy. A change from simple to complex seems very unlikely