changing a record type with records may break those records

[solomonr]

if you change the data type for a field of a record type that already contains existent records, the "record datum" object will still be present, but it's "record datum subclass" object won't match up with the new field's data type.

there isn't really a good fix for this -- i could try creating new datum objects for all existing records and try "casting" the data from one field to another, which might cause problems. even in this case we should warn the user that there may be a loss of data.

what we could maybe do is just warn the user that they are changing an existing field type, say we're going to delete those datum objects, and ask if that is ok.