No there is no way to do this without a script and quite a bit of work. The main reason there is no way to do this is that it's not a good idea. The way you describe this ID field it's probably the primary key of this table. If that's so then it's logical to assume that it's used as a foreign key somewhere else in your database. So if you moved all records that are >N to be -1 then you'd break all of your foreign keys and have no way of getting them back.
Is there any reason that you feel you need your ID field to be in order (1,2,3,4,5...)? It shouldn't matter what that field holds as long as it is unique.
