The updated guidance is about the USCG that decided that:

The existing extensions with a compliance date stipulated as the 'first scheduled drydock after [date]' will NOT be affected
Vessels that have received an extension of the ballast water compliance date that was stipulated as the 'first scheduled drydock after [date]' and subsequently experience drydock slippage, enter a drydock for emergency reasons, or enter for the purpose of installing a scrubber will retain the originally issued extension, provided there is no change to the next scheduled statutory out-of-the-water survey of the ship’s hull for the required inspection of the outside of the ship’s bottom.

In the meantime, a vessel’s statutory out-of-the-water survey for the required inspection of the outside of the ship’s bottom will be verified by vessel records, including the Certificate of Inspection, Passenger Ship Safety Certificate, Cargo Ship Safety Certificate, or Cargo Ship Safety Construction Certificate, as applicable.

Background:

Many are the vessels that have received an extension of the ballast water compliance date that was stipulated as the 'first scheduled drydock after [date]'.

Specifically, the [date] was the date of entry reported to the Coast Guard for the upcoming statutory out-of-the-water survey for the required inspection of the outside of the ship’s bottom.

Based on the schedule for statutory out-of-the-water surveys, these extensions were expected to extend the compliance date no more than five years. Due to drydock slippage, some of these vessels entered drydock after the date originally reported to the Coast Guard.

Additional ships entered drydock prior to 'first scheduled drydock after [date]' for emergency purposes or to install an exhaust gas cleaning system (scrubber), but with no change to the schedule of statutory out-of-the water hull surveys.

Under the terms of the extension letters and the guidance provided in MSIB 13-15, these circumstances substantially affected the original extension period.

The Coast Guard has received multiple requests from vessel owners and operators seeking their extensions be amended due to drydock slippage or entry into a drydock either for an emergency or for the installation of scrubbers, but not for a statutory out-of-the water survey.

The Coast Guard normally responded by granting these requests with a 2.5 year extension from the date of the originally scheduled drydock date.