With NCL if you have "paid in full" and there is a price adjustment downward they will not lower your fare.
If you have not paid the final balance, NCL 'WILL" make an adjustment downward, to reflect the new price.
We are sailing the NCL Star in November for the Panama Canal and have had our price drop 4 times and have saved $1200 each. We have not made the final payment and will not until the required date. If the price continues to drop on our specific cabin grade, NCL will reduce the price to us.
We have NEVER had any disappointments with NCL honoring price reductions on any of our cruises.
We are NCL Platinum Latitudes Members, and have cruised NCL too numerous times to even count. Never had any recorded problems.