It's been a while since I used slide films so things may have changed since then. I always thought the prepaid processing was valid without limit but it's recommended that the film is processed before a certain date. If you don't process it by the date you can expect various degradations, just like if you eat food after its best before date it might be fine, it might taste funny, or it might cause you to spend the next 43 hours sitting on the toilet wishing you'd just thrown it away.
After this long I'd expect to get some seriously surreal colours but there's nothing lost by sending it off.
