I find it difficult to conger up any sympathy for your terrible "shafting". Your post is a little lean on facts, but I assume you are doing a RT Portland ME and St Thomas USVI on Continental Airlines (I thought Con Airlines was that convict airline thingee). I assume you are taking the ferry to/from St John. You picked one of my favorite islands. Smart choice. Nice reggae there.
I seriously doubt CO makes its flight schedules and changes to coincide with your reserved flights. I know they dont do such re me and my 100k mi/yr. Changes happen all the time. IT IS A FACT OF LIFE. Sometimes they are in your favor, sometimes not. IMHO you were fortunate to get 3-4 freebie seats. I never could on CO, DL, UA etc. Also the free hotel too. As for timing, it appears you made reservations approx 6 mo in advance. A lotta things can change in 6 mo.....ie just look at the NASDAQ or Firestone tires. Once I planned a trip on CO to Bali. They dropped the service. I got over it in 2 hours. I realized no matter how much I whined, they were not going to restart it.
It always pays to be PROACTIVE and not reactive. Phone calls are cheap (especially to 800 numbers). Do it and you will have nicer trips (and a nicer life in general).