Your organization's data is never automatically deleted when you change plans. What changes is access to certain features.
When upgrading
All historical conversations are preserved and remain accessible.
Teams, playbooks and specific fields already configured keep working.
Features from the new plan are unlocked immediately.
Open commitments aren't affected.
When downgrading
Historical conversations: preserved in full. They aren't lost or anonymized.
Open commitments: remain active.
Teams and playbooks: when you downgrade to Starter, every team except the default team is deactivated, along with every playbook that isn't in a shared category. They aren't deleted.
Roles: everyone becomes a Leader of the default team.