You will definitely find Windstar to be different from Windjammer. Though Windstar is casual, it's not the ultra-relaxed atmosphere you find on Windjammer. In other words, passengers need more than a clean t-shirt and bare feet in the dining room at dinner!
That said, on Windstar you will find a relaxed, elegant atmosphere with wonderful food and service. I thoroughly enjoyed my Caribbean cruise last year. There's nothing much in the line of entertainment, so boredom could present a problem on a long repositioning sailing. Be sure to bring lots of reading material. The ships have a decent CD and video lending library, but if you have any particular favorites you might want to take them along.
If you're looking for R&R and pampering, you'll have a wonderful time.