If you are Gold/Platinum, you have to call in (or have the TA do it for you). But Silver or first timers can simply do it online. Much easier than calling.
That said, March 2017 when/where matters. If you are looking at a cruise during school days, and/or a 3 or 4 night, there's nothing for you to worry. These don't change the prices for a while.
The ones that fill up are the new itineraries or those during school holidays (xmas, new years, spring break, summer, etc).
Regarding transferring after to a TA, you can do this up to 30 days after making your reservation, however, you will more than likely loose some/most/all of your on board credits (typically it's half). So if it's a long cruise where you would get a considerable OBC, you should contact a TA ahead of time and have them switch it for you on opening day.