Disney tickets don't lose their 'value', even after they expire. If there's been a price increase you simply pay the difference. Per Disney's website: "The amount paid for an unused, expired ticket may be applied towards the purchase of a new ticket at the current price so long as the new ticket purchase price is equal to or greater than the amount paid for the original ticket.". If you're buying 3rd party be sure their tickets are subject to the same policy.
That being said, tickets are generally NOT refundable by Disney. An exception to this is if you book your tickets through Disney as part of a package. The ticket price is refunded if you cancel the package. We live clear across the US from WDW and this is why we always book a package through WDTC. If for some reason we have to cancel and it looks like we may never make another trip to WDW, we aren't 'stuck' with tickets we will never use. We may pay more, but we look on it as 'insurance'. This may be something to think about if you're coming from AUS unless you're 10000% sure you'll be coming to DLR, even at a later date than originally planned.
Some 3rd party sites might also have a refund program, but you'd have to check with them.
As far as when DLR may increase prices, no one knows anymore. It used to be that price increases came in the Fall or after the 1st of the year (Feb?) but that changed a few years ago and now they happen when they happen with no fore-warning.
