A problem that I have seen crop up a few times recently is the scenario where you have created an IDENTITY column as an INT, and now are nearing the upper bound and need to make it larger (BIGINT). If your table is large enough that you’re hitting the upper bound of an integer (over 2 billion), this is not an operation you can pull off between lunch and your coffee break on a Tuesday. This series will explore the mechanics behind such a change, and different ways to make it happen with varying impacts on uptime. In the first part, I wanted to take a close look at the physical impact of changing an INT to a BIGINT without any of the other variables.
Read Article series by Aaron Bertrand who provided different solutions with analysis – http://sqlperformance.com/2016/01/sql-indexes/widening-identity-column-1