There are actually three questions that are open.
The one seashoreCM raised is the most likely, IMHO, to be answered in the negative, i.e., that when you add days to a pass, you'll not be charged difference between the cost of your pass as if you just bought it and the current cost of a pass with the number of days you want, but rather the difference between the original cost of your pass and the current cost of a pass with the number of days you want.
The other two involve park hopping and the non-expire option, with respect to whether either, or both, is considered an upgrade, and therefore wouldn't result in your basically losing the benefit from having purchased passes at 2005 prices. The guesses regarding that seem to be trending "positive" that for both of these things, you'll be charged based on current prices, rather than the price you actually paid.