Just to clarify a common misunderstanding...
At the time you book and select a stateroom, there is no GTY available. GTY rates come out after a certain percentage of staterooms in a given category have been booked and selecting a stateroom in that category is no longer offered. The GTY rate may actually be higher than what most guests paid. So the statement above is essentially suggesting that
DCL charge more for a rate that wasn't available when people booked. DCL is moving people to the same or higher stateroom category as they booked -- so if you booked a 4B you'll be in a 4B or 4A room (or concierge).
I think you are confused with the more generic but discounted and restricted *GT rates, which allow DCL to assign any interior, oceanview or verandah stateroom. While discounted off the prevailing rates, it may not be less expensive than opening day rates for those who booked early. *GT aren't offered on every cruise. If you want everyone changed to a *GT rate, it's a broad category so a 4B could be bumped
down to a 4C or even a 6A.