couple of years Disney has raised ticket prices in early August. I think it has been the first weekend recently. The price increase is announced on Friday and implemented on Sunday.
The amount varies by ticket as different features may increase by different amounts. A couple of years ago we were informed of a pending price increase and bought our tickets ahead of time. The particular tickets we bought, 10 day park hoppers with no expiration, increased a little over 9% in price. The biggest increase was in the no expiration feature. The other areas increase three or four percent.
You can buy tickets ahead of time and the 14 day window for changing your tickets does not start until the first use. We bought ours in August and used them the next June. On that trip we added the Water Parks and More option.