If you change the data type of any custom field that is used for lead conversion, that lead field mapping will be deleted. If you change the data type of a custom field that is set as an external ID, choosing a data type other than text, number, or email will cause the field to no longer act as an external ID.
The option to change the data type of a custom field is not available for all data types. For example, existing custom fields cannot be converted into encrypted fields nor can encrypted fields be converted into another data type.
https://help.salesforce.com/apex/HTViewHelpDoc?id=notes_on_changing_custom_field_types. htm&language=en