It's Standard practice throughout the industry. If the ships finished early it costs money to keep it in dock (port charges) so off it goes on a jolly & you will have no chance of compensation.
I would be glad that this turned out not to be the maiden voyage, hopefully any teething problems may be ironed out by the time you board.
I have never understood the kudos of going on a maiden voyage. Far better to go on the ship in its first year and get the benefit of the crew learning by their past mistakes and you having smooth service and facilities. Oh! No you will not get compensated but I hope a pleasant cruise. .... Neil.
I "was in the same boat" (excuse the phrase) earlier this year with the Independence of the Seas "maiden" voyage which was supposed to be mid May, however, RCI brought this forward to the beginning of May with a series of additional "maiden" voyages.
I was offered the chance to change at no extra cost, or to retain the original date with no compensation. They would not budge from this stance, and I believe this is an industry standard for this type of change. They are quick to quote therir T&Cs!