Why would you want to replace a bearing that isn't showing any problems? Headsets wear out over time, and there's no danger or negative consequences to waiting until there's a problem, such as indexing, or unsmooth steering.

If the cartridges aren't sealed you might flush them clean and regrease from time to time to extend their life. Also if they're identical you can switch top for bottom since the bottom gets 90% of the wear.

Otherwise enjoy something that works, and don't replace it until it doesn't anymore.