Things don’t always go as planned. This quote can apply to many topics, including sizing up your data types. When you started out with a new table, there were no concerns with SMALLINT—until you realized you needed INT. Or maybe you were rolling along fine with INT until you needed to up your game to BIGINT.
If you don’t have much data in your tables and you need to change data types, you may not have much to be concerned about when it comes to the impact of such a change. But problems can arise when you have a table full of data and want to shave off some time to make a data type change as smooth as possible.
The secret here is that, when you’re using row compression, SQL Server maintains the smallest number of bytes for the numeric data. For example, if you have three bytes of 0 and one byte of actual data in your int (i.e., the number is between 0-255), row compression will store one byte. If you subsequently bump this up to a bigint, row compression stores one byte, whereas default non-compressed pages need to be re-written to store seven bytes of 0 and one byte of your number.
This can be a really smart idea if you’re planning a data type change on a large table, as there are extremely few cases in which row compression is bad (“extremely few” as in “I’ve never seen one myself but I have to hold out the possibility that it could happen”) and if you have Enterprise Edition, you can rebuild all of those indexes online and ensure that you aren’t blocking the world for very long.