Depending on when you purchased the ticket, it may be better to just use the remaining days on it OR buy a brand new AP and keep the old ticket for some future visit.
Why? That's because the cost of the upgrade to AP will be calculated by taking the price you paid for the 10-day pass, multiplying it by 60% (representing the value of the remaining 6 days) and subtracting that amount from the current price of an AP. So, if you bought the 10-day pass a while ago, your price per day is probably lower than today's prices. You'll have to do the math and consider your own finances to determine what's best for you.