We replace pads and battery as we cannot afford to but a new one. The battery was much less expensive on Amazon than any other place, including the SuperStore. I do know some places that have the same AED as the local ambulance will swap pads that will expire soon with them as the ambulance folks will use them before they expires.
A Zoll customer service rep. told me Zoll AED's will still function past the warranty end date as long as the AED has the most current software update, current batteries and pads. Zoll sends an email when there is a new software/firmware update available. I don't know other AED manufacturer's recommendations on AED replacements or upgrades.
AEDs are so expensive we will not be replacing ours until it doesn't work anymore or pass inspection. I buy replacement batteries and pads through the AED superstore. The state office of emergency planning has been very helpful in the past with assistance for our AEDs.
I just replace pads/batteries even after warranty expires. Zoll has an trade-in / upgrade program.