Amendment of customs documents allowed by officer subject to prescribed conditions; post-clearance changes require pre-existing documentary evidence. Amendment of customs documents is permitted at the discretion of the proper officer subject to prescribed form, manner, time, restrictions and conditions, except as otherwise provided; amendments to bills of entry, shipping bills or bills of export after clearance or export are barred unless supported by documentary evidence existing at the time, and authorisations or electronic amendments may be effected through the customs automated system or by importers/exporters on a common portal as specified.