Danny Kruge shows one way to change a table’s identity value from integer to bigint:
The table was around 500GB with over 900 million rows. Based on the average number of inserts a day on that table, I estimated that we had eight months before inserts on that table would grind to a halt. This was an order entry table, subject to round-the-clock inserts due to customer activity. Any downtime to make the conversion to BIGINT was going to have to be minimal.
This article describes how I planned and executed a change from an INT to a BIGINT data type, replicating the process I used in a step by step guide for the AdventureWorks database. The technique creates a new copy of the table, with a BIGINT datatype, on a separate SQL Server instance, then uses object level recovery to move it into the production database.
There’s a way to do this without any downtime, though the trigger logic gets a little more complex and it does take longer.