There are numerous synonyms that can be used in place of the phrase "in stages." One such synonym is "gradually," which means something is happening slowly or in small increments over an extended period of time. Another option is "stepwise," which refers to a process that occurs in several planned or incremental steps. "Phased" can also be used to describe a process that occurs in stages or steps. Additionally, "incremental" can be used to describe something that is done in small, gradual or incremental steps. "Progressively" is also a suitable synonym, referring to something that happens little by little over time.