Here is my tough love message:
You are asking them to cancel and rebook you cruise to another date, even if you don't agree with them. Your situation is not unique. It happens hundreds of times each week for the major cruise lines. People want to move their cruise around like a dental appointment. If the cruise line allowed this to happen without a penalty once inside the contractual penalty phase they'd quickly have chaos in their reservations system.
Next time use an agent. They may have enough pull with the cruise line to get the cruise moved without a penalty.