The cabins themselves are the same size (191 square feet) but the balconies are not. The balcony on the Caribbean Princess is 40 sq ft, the one on the Ruby is 73 sq ft.
The Caribbean P is going into dry dock this coming May for a two week freshening up, so that is good for your revised cruise.
While your particular cruise may have been selling well, they could have had a lot of open cabins on the Caribbean P, meaning they had excess capacity for Caribbean sailings for those months. At the same time they saw interest in South American sailings with higher pricing than they could get in the Caribbean - so the Ruby gets moved.
The Caribbean is very dynamic for the cruise industry these days, with excess capacity all over the place. It's a major reason Royal Caribbean is moving some of their biggest ships back to Europe in 2015. I could book a 7 night cruise on the MSC Divina out of Miami for under $300 per person right now. Certainly not in the same league as Princess, but still it's a week in the Caribbean for under $100 a day.
This is a schedule
of Princess refits, but it isn't very complete as far as information on what they will do.