Hi Sarah! :wave We were just on the Coral last month, the April 1st Panama Canal sailing. We're planning to sail on the Golden in your neck of the woods next July.
The Caribe balcony cabins have the largest balconies. C406 is a good mid-ship location and I highly recommend keeping a mid-ship location!! Our tablemates had cabins at the bow of the ship - around D250ish - and said they were kept awake all night from the pitching and rolling of the ship. Now, we were in 3.5 foot waves!! (If you can call those waves!) We were mid-ship on the same deck and hardly felt a thing.
The only problem with your travel agent putting "will accept upgrades" on your booking is that you could end up at the very back or front of the ship. Or, you could end up in a mini-suite with no balcony whatsoever. (The Coral has different deck plans than the Golden as you've probably noticed.)
If I were you, I'd keep what I had unless prices drop like they did for us. About 5 months before our sail date, it was actually CHEAPER for us to book 2 mini-suites than it was to book 2 Category BA's on the Caribe Deck!! We saved about $800 by switching to mini-suites. I know that's weird and not the normal way things go, but it was the same for our friends' sailing on the Island Princess the following week.
Keep an eye on prices at a few different websites. When you see a price change, call your TA immediately. If there's a less expensive MID-SHIP mini-suite that's available, get it then. Don't be at the mercy of Princess and let them decide where to put you unless you're one of those lucky few who are very flexible and don't care where they are on the ship!
Good luck and let me know if you have any other questions!