Inserting Missing day of data into table - Need to increment primary key by 618 for each key
Since you're changing the identity of the row, can we assume that it's not used as a foreign key of another table? If it's only purpose is to uniquely identify the row of the table it's on, then don't bother attempting to assign it a new identity because it's already auto incremented by one, upload your CSV without specifying the id column and it will take care of the id. 

You'd have to worry about key collision and identity_insert etc, it's just a mess, but if your heart is set on it, there are ways it could be done.