Absolutely, positively DO NOT go to guest services before using that ticket!!
Use that ticket first! It is completely upgradeable for 14 days after first use, even if the days are "used up". Chances are, you purchased that ticket before last years price increase and will use it after this years upcoming price increase (without your actual dates I can't know for sure, but it's pretty darn good chance).
By using it first, it will be worth the *current* gate value of a one day ticket and therefore you need to only pay the difference between that and what the current ticket you want is. If you do not use the ticket first, you will pay the difference of what you paid for it and what the current gate value of the ticket you want is. And since prices went up ~5% last year and probably ~5% this year, that's a good amount of money.
Of course you may upgrade it to any ticket that costs more than the ticket you have at current gate value (since you WILL use it first).