Custody agreement from 6 years ago doesn't work anymore

Quick backstory: My kids are now 12 and 14. Our custody agreement from way back said EOW and alternating weeks in summer. But my oldest wants to do clubs at school, my youngest has been getting into sports on specific nights. Both things that don't align with our schedule at all. Had a conversation with my ex about being more flexible, less structured—just whoever has them handles the logistics and we keep them in activities—but she's worried about 'abandoning the agreement' even though neither of us are actually fighting about it anymore. Do we need to go back to court to modify it or can we just agree to something new in writing between us? Any lawyers in here?