Consider to use SEQUENCE instead of IDENTITY.
IN sql server 2014 (I don't know about lower versions) you can do this simply, using sequence.
CREATE SEQUENCE sequence_name START WITH here_higher_number_than_max_existed_value_in_column INCREMENT BY 1;
ALTER TABLE table_name ADD CONSTRAINT constraint_name DEFAULT NEXT VALUE FOR sequence_name FOR column_name
From here: Sequence as default value for a column