Your ideas don't have to be really that different from everyone elses, it just has to answer the prompt.. what is your prompt, by the way?
Anyway. Lewis changes in three ways.
1) His personality changes; at the start of Cosi Lewis was naive, not confident and a pushover, through his experience he changed and he had more resilience (e.g he could stand up to Nick and Lucy), he had confidence and a better understanding of life,
2) His values and motives change. His view on love ends up being more about love than lust by the end of the play. At the start he was unsure about love but by the end he knows what he values (faithfulness). Lewis's understanding of the mentally ill also changed as he first held very stereotypical views of the patients but by the end this changed.
3. His relationships changed as well. Pretty self explanatory.
Good luck!
