You can only upgrade a non-elite when they are travelling with you, and it is limited to one additional person. Otherwise, you cannot do what you'd like to for your parents unless you actually pay the difference between the fare they paid and the actual J-class fare. Since their flight is part of a cruise package, the difference will likely be $1,000 or more each. AC has no upgrades for points, nor will they take UA certificates (unless the flights are on UA codeshares and of course you are accompanying the travellers).