I tested this on our development server, but USE AT YOUR OWN RISK.
To figure out the moduleID of your Form and List module, you can try the following script (from within SQL management studio (SMS)):
SELECT UserDefinedFields.ModuleId, UserDefinedData.UserDefinedRowID, UserDefinedFields.FieldTitle, UserDefinedData.FieldValue
FROM UserDefinedData INNER JOIN
UserDefinedFields ON UserDefinedData.UserDefinedFieldId = UserDefinedFields.UserDefinedFieldId
where UserDefinedData.FieldValue like '%Labor%'
order by UserDefinedData.UserDefinedRowID desc
This will list all Form and List module items which have "Labor" in their field name. This will help you track down your moduleID; in this example '3770' is the moduleID of the module I need to edit.
Once you do this, you can then use this script to figure out the UserDefinedFieldID for your appropriate column(s); again using SMS.
SELECT *
FROM [UserDefinedFields]
where moduleID like '3770'
Once you figured out the UserDefinedFieldID ('1113' in this example) of your columns, you can change the FieldType by doing the following SQL script within SMS:
Update UserDefinedFields
Set FieldType='EMail'
where UserDefinedFieldId like '1113' and ModuleId like '3770'
This will update this field from whatever it was to an EMail column type.
Hope this helps you out..