What's the best strategy to preserve vintage headset life? I recently purchased a complete Dura Ace 7400 set (8 speed, downtube shifters so 7402 parts) with the 7400 headset as well. It's the improved one with the silver race and not the gold one prone to bearing brinneling. So far, I haven't noticed any problems in the races. The headset came with the original bearing retainers  (7400 was races or loose ball, 7410 is cartridge).

When vintage headsets do have brinneling, it's recommended to replace the retainers with loose balls to give the headset a second life. In order to keep my headset races functional for the longest time, should I put new bearings in the retainers and install, and then in many years when I need a headset servicing replace those retainers with new loose bearings; or should I skip right to packing new loose bearings now and potentially a second time in the distant future? (The retainers will live in my toolbox so I can have it be "original.")

I've heard people recommend rotating your headset races as a potential solution to mitigate bearing brinneling. Is this relevant or helpful in this situation?

The frame is made by Stowe in the late 80s. While not very historically significant, it's a vintage piece  I want to preserve and keep in good condition. I will probably not ride it in regular rain or any races. I'm a pretty gentle rider and weigh 145 pounds.
I'm not sure why you're asking this, because as far as I can see the answer is in the question. You want to "keep it functional for the longest time" and then you say "should I discard a part that extends the life"?

Ball bearings without separators/retainers wear out faster, so there's more metal fragments in the race and the whole unit wears out faster. A much better way to extend the working life would be to extract and preserve the original retainer, and use new retainers and bearings. If you replace those more often than strictly necessary you'll minimise wear on the races.

But remember that the headset is very rarely the first part of the bike that wears out. Unless the bike is stored in atrocious conditions or terribly badly adjusted, by the time the headset wears out the derailleurs are normally gone, you're on the fifth to tenth bottom bracket, third or fifth wheelset and so on. A better question might be whether you have a stock of those parts to use as spares in order to maintain your bike in original specification. But note that this only applies if you ride the bike, and since you're asking about keeping it working for time rather than distance, that may not be a factor.

To keep the bike functional for the longest possible time you should dismantle the bike and pack all the metal parts in heavy grease to preserve them. The plastic and leather parts are more problematic, they probably need to be chilled and kept in an oxygen-free environment. That should allow it to be reassembled and used at any time in the next 100 years or more. If you choose a good storage location (a dry cave, for example) and a proper container (a sealed stainless steel case, perhaps), extending the functional life by 1000 years isn't out of the question.