Lots of postings on this topic, but the answers should be pretty clear by now.
Pay ahead by cash, check, direct bank withdrawal, etc. etc., any of the methods that do not cost GCT/OAT the usual 3-5% credit card loss and you are in the Good Buy plan and won't see an increase. Pay by credit card, in full up front or part now part later, GCT/OAT loses 3-5% to the Visa et al Bandits, you may get miles from the Visa Bandits, but in return you agree that GCT'/OAT may increase your price for any reason prior to departure. I just went back and re-read the "General Terms and Conditions" on the website and, although the wording requires pretty careful parsing (that's one of the things I did for a living before I retired), yes, they can increase the charge at will.
"Operator reserves the right to correct promotional or pricing errors at any time, or to increase the Trip Price in the event of cost increases due to changes in airfares, cruise fares, currency fluctuations, park fee increases, taxes, or fuel surcharges, or other reasons, unless you pre-paid according to the terms of the Good Buy Plan prior to the cost increase going into effect."
To be clear, next month is my seventh GCT/OAT trip, I've always paid in full by credit card on the day I first booked, and I've never had them bump the price later. So, no, I don't think it's something they do as a bait and switch. On the other hand, times change, and maybe they've revised their business plan to our detriment. Time will tell. In any case, it would have been much less of a PR disaster had they e-mailed everybody the moment they posted it into your online accounts, rather than let you discover it when you viewed your statements.
One poster mentions hedging. This is hedging, on your part and on theirs. You pay cash-equivalent ahead and they protect you from increases. On their part, you pay cash-equivalent ahead and they get the float on your money and don't get robbed of 3-5% of the payment by the Visa Bandits, but they also can't increase your trip price if their costs go up. You are their hedging counterparty, in business lingo.
As an aside, I have to admit to a certain (very) mild resentment when I see people who could afford to pay by cash or check using their cards instead simply to get the miles, because I know the merchant is paying the Visa Bandit, and he has to charge everybody more (me). The flip side is that so many credit card payers also pay huge fees and usurious interest to the Visa Bandits when their credit card statements come.
And yes, I did pay by credit card, and I know I'm at risk for an increase. But my next trip was just booked within the past week at a truly humongous last-minute discount, after the increase you're noting, and departs in five weeks, so I've likely escaped this one. My online statement still says paid in full. The OAT rep offered the $50 Good Buy plan discount with price protection on the phone when I booked, but I declined in favor of the credit card convenience.