I'm not sure what you mean by "cash it in". If you mean can you get a refund on it, no.
If you mean can you apply the value towards a ticket upgrade, yes but you really wouldn't want to. Your ticket from 8 years ago will still get you admission to the parks for however many days remain on it with no additional cost to you to use it.
But if you upgrade it, you will be taking what you paid for those admissions 8 years ago and paying the additional cost to get the same thing back 8 years later at gate prices that have seen at least 7 price increases since you bought your ticket.
Example: You have an unused 5 day PH. You paid $200 for it 8 years ago (ficticious price used as an example). That means your daily cost of admission was $40. You trade it in now to upgrade to a 7 day PHP. You will get the $200 credit and have to pay an extra $150 to get the new ticket. That means your daily cost of admission has risen to $50 per day for the entire 7 park days. What you actually did was pay an additional $50 (5 days x $10) to get back what you already had.
Quite honestly, your best bet with a ticket that old is to just plain use it as is and forget about upgrading it. You can't come out ahead any other way.