When a promotion is changed, its old version is saved and if there are any subscriptions a promotions was applied to before a promotion properties were changed, these subscriptions keep using the old version of a promotion. Thus, any changes made to promotions do not affect subscriptions a promotion is applied to. When you edit a promotion, you simply create a new version of it.
Old versions of promotions cannot be changed. They can be deleted only.
It is possible to see, if there are subscriptions exist an old version of promotion is applied to. If no subscriptions use a promotion old version, you can delete such version.
To view the versions of a promotion: