An "immediate upgrade" as you call it is just giving you a second chance to buy the Business Class seat at the going rate that you neglected to take up when you first bought the flight in Economy Class. (By default, Aer LIngus shows you the price for Economy and Business on all sectors, allowing you to choose from either cabin as desired. If you selected a paid Economy ticket, it means you were shown a Business Class fare that you rejected. This will merely allow you to change your mind and select that fare)

It's not going to be any cheaper. It's merely giving you the option to change your mind and buy Business Class proper at the normal Business Class price.

(If you want to travel in Business Class and earn Business Class Tier Points, you gain absolutely nothing by going via this buy Economy, upgrade later route - you end up paying the same* as if you had just paid for Business Class outright at the start)

*fares can fluctuate so the price you pay may be more, or a little less, than the Business Fare that was shown to you when you opted instead for Economy; but you're not getting a discount or deal when you end up in Business by this route. The "discounted" ways of ending up in Business - bidding for a seat (which bid they may or may not accept), or using miles, means you don't get the Tier Points for business. You only get Business Tier Points when you pay full-fare for Business Class.


The above applies to cash tickets purchased directly from Aer Lingus. As others have mentioned, you cannot upgrade award tickets. Your only real option here would be to cancel your Economy Award ticket, and seek a new Business Award ticket. (Obviously, don't cancel your Economy ticket if there is no Business ticket available for you to redeem into outright)