You can specify a <rollback>
block (docs) in your changeset to describe how to roll back the change. Within your rollback tag you can use raw SQL or a <createTable>
tag to re-describe what the table looked like before it was altered.
You can also specify the changeSetId and changeSetAuthor in the rollback tag to point to an existing changeSet that will recreate the table. This approach can be easier if there has been no other changes since the object was created but doesn't work as well if there has been multiple changeSets that modified the object since it was first created.