To change values of existing integer field (`table` is the table name, and "id" is the column name), you may use the following update SQL command:
update `table` set id=id+10000;
Let's say that you have an ecommerce website, and you want your order number to begin 10001. But, you already have a handful of orders already so you'll just update the existing values.
If you're updating your primary key values, MySQL will automatically adjust auto_increment value to begin after the largest number stored in the database.