This is a very disruptive change to the structure of the table, obviously. (And an interesting side observation: the physical order of the columns, RowID and filler, have been flipped on the page.) Reserved space jumps from 136 KB to 264 KB, and average fragmentation bumps up modestly from 33.3% to 40%. This space does not get recovered by a rebuild, online or not, or a reorg, and – as we’ll see shortly – this is not because the table is too small to benefit.
Note: this is true even in the most recent builds of SQL Server 2016 – while more and more operations like this have been improved to become metadata-only operations in modern versions, this one hasn’t been fixed yet, though clearly it could be – again, especially in the case where the column is an IDENTITY column, which can’t be updated by definition.
Read the whole thing. The clustered key scenario (which will be later in the series) is a bit more interesting to me, as that would be a more common use case for identity values.