There is no way to modify the identity property of a column through scripts.
You can try doing it from enterprise manager and generate the change scripts it follows the steps as below

- create a tmp table with same design
- transfer data to the tmp table from original table
- drop the original table
- create with new specs and
- retransfer data with identity of
- switch on the identity

however do ensure that the column you want to change to identity has no nullls and duplicates

Regards
AM