This is a very major change that I was not really aware of. If I understood that NVIC correctly after a quick scan:
It takes 36 months of sea time sailing as OICNW, or 24 months of sea time, if it includes 12 months as Chief Mate, to upgrade to from 3rd Mate Unlimited , STCW II/1, to Master 500 or 1600, STCW II/2 500-3000 GT.
It also, basically requires most of the same STCW courses as STCW II/2 unlimited.
So while a 3rd Mate with II/1 can still upgrade with only 12 months of seatime to 2nd Mate and use the same II/1, and he can upgrade to national Master 1600 Oceans (same as before), he cannot get STCW II/2 500 to 3000 GT.
This would limit the 3rd Mate to Master 1600 Oceans guys with less than 36 (or 24) months of seatime licenses to only being able to sail on “Inland” waters, i.e., inside the Inland/Colregs boundary line (essentially, the Great Lakes fleet and a few ferries, and STCW exempt fishing vessels).
I see two possible effects: first, this should slow the flood of 20-something Master 1600s into the very crowded 1600 ton job market, and second, it should discourage the construction of self-propelled vessels over 200 GRT (which require STCW) and encourage the tug and barge trade.