Any stateroom can be upgraded provided there is room available. Basically, there's a chart of prices of upgrading from your current category to the one you want, and this will be at a discount.
But YMMV with *GT rates in terms of what you pay. For example, if you booked IGT or OGT, you may end up with an outside or even a verandah. Technically, the upgrade cost would be from the lowest category you were booked on (ie 11C if you booked IGT), but some were upgraded from the category they were assigned. And this could be a big difference, specially if you got bumped up a bit.
To answer OPs question, if you book IGT or OGT, there's a possibility you would be bumped up to the next type of stateroom. Last August, on our first cruise, we booked OGT. We were assigned a cat 5e stateroom. We were bumped up. Most of the folks that booked VGT were bumped to a cat 4. OGT were moved to cat 6 or 5. IGT were moved to oceanview. But of course your mileage may vary... OP: You need to decided on the type of stateroom you are willing to accept - inside, oceanview or verandah. Then you can hope to be bumped, and even look for an upgrade at port.
Last year, I knew of someone that booked IGT. Got bumped to an oceanview, and then at port upgraded to cat V.
