AFAIK exising users will get the new default value assigned, when they update their user profile. You may create the value in userprofile table using SQL in Host menu:

Lookup propertydefinitionId (replace yourPropertyName with your value and modify portalId if needed):

SELECT [PropertyDefinitionID]
  FROM {databaseOwner}[{objectQualifier}ProfilePropertyDefinition]
  WHERE [PropertyName] LIKE 'yourPropertyName' AND [PortalId] = 0

now you may insert your new values for all users, who don't have a value already (modify portalId if needed):

MERGE INTO {databaseOwner}[{objectQualifier}UserProfile] P 
  USING {databaseOwner}[{objectQualifier}UserPortals] U ON (P.UserID = U.UserId AND P.PropertyDefinitionID = 17)
WHEN NOT MATCHED THEN INSERT (UserID, PropertyDefinitionID, PropertyValue, Visibility, LastUpdatedDate)
 VALUES (U.UserID, 17, 'Your Default Value', 1, GetDate());

You'll need to restart the application to reload all user objects.