If they are classified as simple cysts they are by defintion are, and will remain, benign. The only way they will "get worse" is that they may get larger or more painful with hormone flucuations, or in some people, with ingestion of caffeine-containing products (coffee, colas, chocolate, etc.).
If they are suspected to be complex cysts, a biopsy maybe recommended, just to be on the safe side.
Did the ultrasound report contain a BI-RADS score, or any recommendations regarding follow-up?
Best wishes...
Cysts are usually benign findings; they can certainly increase in size since they are fluid filled and they can also decrease in size as often there is some relation to hormonal levels. Whatever recommendation was made following the Ultrasound should be followed, if anything was advised. Regards ....