Cell theory will never become a law of cell. This is because theory and law are two different things and one can not develop and become the other.
When a scientist observe a phenomenon to which he did not have an answer, he will formulate an hypothesis about the observation and set out to conduct an experiment to confirm his hypothesis. If his hypothesis is confirmed, then the hypothesis become a theory, especially after it has been confirmed by other scientists too.
But a law refers to a natural phenomenon which have been observed to occur in the same fashion, for a very long period of time. For instance, the gravity force will always force an object throw up to come down. That observation is part of the Newton's law of motion.
