First, you need to talk to your agent to find out the type of fare code used for your booking. If you booked something like the Early Saver Fare, then there will be a $50 fee for changing to another booking or you'll lose the entire deposit if you cancel. This applies if you do it before the final payment due date.
However, if you booked a regular fare code, then there will not be any fees if you change or cancel prior to the final payment due date.
As for the availability of cabins, it all depends on how well that particular itinerary is selling. If they've sold alot of cabins, then obviously all the good ones will be taken and all the cheap prices will be gone. Conversely, if sales are slow, then specials could be had and good cabins may still remain.
If you need the deposit money now, then it may be best to cancel provided you do not have a fare code that has fees involved. If there are fees involved, then it would behoove you to wait until just before final payment due date and then make a decision whether to change or cancel.
But talk to your agent as they can provide specific information about your particular reservation.