Shortening the length of the field from 18 to 13 will indeed chop off the last 5 characters.  If they’re positive that there is no useful data past the 13^th character in that field on any document it has been applied to, then shortening the field length is a safe way to fix it.  They should be careful though, because making the field shorter is irreversible, and will cause permanent loss of any removed characters.