Hello anonymous user
You can upgrade or downgrade to and from any Azure API Management Service Tiers. It does take 15-45 minutes to complete the upgrade/downgrade and you can read more about this at https://learn.microsoft.com/en-us/azure/api-management/upgrade-and-scale#upgrade-and-scale.
If you are going to or from a Developer tier, there will be downtime during the upgrade/downgrade. You can read more about this at https://learn.microsoft.com/en-us/azure/api-management/upgrade-and-scale#downtime-during-scaling-up-and-down.