Using the metadata deploy() method or running a package upgrade and either of these deployments contains the conversion of a custom field from one data type to another, an error will be thrown. The error occurs if a deployment changes the data type of one or more custom fields and the objects that contain the affected custom fields contain a large amount of data. The limit is 85 million custom field type conversions
How: In a deployment, a custom field is converted from text to picklist, the deployment could fail with this error as the data type of one or more custom fields cannot be changed. To resolve this error, see this knowledge article
