Just stumbled on this. We just had to fix the same issue in our environment a few weeks ago. Our scenario was a bit different where we had a very granular partitioned table and we had to rebuild indexes as well.
We followed the same steps but we did it all in T-SQL. Basically what we did was switch out the load data into the new table 1 partition at a time into a staging table (no identity column), rebuild indexes and switch it into the new table (with identity column). We found that there was much better performance without the identity on the staging table.