Most payment gateway do not allow order id to be re-used. i.e. order ids cannot be duplicated when being submitted to payment gateway for processing. For our case, we were migrating from Drupal 6 to Drupal 8 for one of our clients and we decided to re-start our order ids from 40001, having already used up to over 30,000 in the old site.
alter table commerce_order auto_increment = 40001;