What should be considered before changing the field type of a custom field?

Changing the field type of a custom field may result in data loss if the new field type is incompatible with the existing data. For example, changing a text field to a number field may cause the loss of alphanumeric values. The field type can be changed for most custom fields, but not all. For example, changing a master-detail field to a lookup field is not allowed. The field being a required field does not affect the ability to change the field type.