Dug720 is correct -- if you modify your current reservation to a different category (same ship, same cruise date, same passengers) then the fare should be the price of that new category on the date you originally booked your current reservation. If you simply called to ask "how much is category X for passengers 1, 2, 3, 4" -- they will give you today's rates. Did you use a TA? The TA should be able to check for you. If you didn't use a TA, you should be able to check it yourself as Doug720 mentions. The good news...if you go ahead with the upgrade it could be less than whatever you were quoted over the phone.
Enjoy your cruise!