solte a restrição padrão na tabela


declare
@table_name nvarchar(256) = 'foo',
@col_name nvarchar(256) = 'foocol'

declare
@Command nvarchar(1000)
select @Command = 'ALTER TABLE ' + @table_name + ' drop constraint ' + d.name
from sys.tables t
join sys.default_constraints d on d.parent_object_id = t.object_id
join sys.columns c on c.object_id = t.object_id and c.column_id = d.parent_column_id
where t.name = @table_name and c.name = @col_name
if @Command is not null
execute
(@Command)