Add missing sequence alteration step

I was alerted to this omission by e-mail from Jeff Nettleton.

When I was performing the change at Silverfin, the sequence
was actually created as bigint so this was not needed.

However, this is not guaranteed so it should be mentioned for
completion.

_posts/2022-02-23-zero-downtime-migration.markdown

@@ -1,7 +1,8 @@
 ---
 layout: post
 title:  "PostgreSQL zero-downtime migration of a primary key from int to bigint (with Ruby on Rails specific notes)!"
-date:   2022-02-23
+date: 2022-02-23
+modified_date: 2023-10-10
 categories: articles
 tags: postgresql rails
 ---
@@ -232,6 +233,12 @@ ALTER SEQUENCE table_id_seq OWNED BY table.id;
 ALTER TABLE table ALTER COLUMN old_id DROP DEFAULT;
 ```
 
+If the sequence was created as an `int` sequence it will have an int value for its `MAXVALUE`. To take care of that run:
+```sql
+ALTER SEQUENCE table_id_seq AS bigint;
+```
+If the sequence is already a `bigint`, this will not do anything so it is safe to run just to be sure.
+
 Now finally you can move the primary key from the old to the new field. Normally adding a primary key constraint requires an underlying unique index to be built. However, you can tell it to use an existing unique index which makes the operation a fast metadata only change. This is also why it was important that we built this index before:
 ```sql
 ALTER TABLE table