Since upgrading to 5.3, we've noticed that Pledge amounts can be edited, which then deletes any payments made toward the pledge.
In earlier versions, the Pledge amount could not be edited. (this is true with 4.6; I'm not sure about other versions)
The only discussion I can find is a patch from a few years ago created for 4.7: https://issues.civicrm.org/jira/browse/CRM-17762