First of all you're operating on a few false assumptions.

1- unless you're planning a long trip, or the rims are already showing stress cracks, or brake track wear, the only thing you're preventing is the need to rebuild a wheel, by doing it sooner. Wait until the wheel needs rebuilding as evidenced by an actual problem.

2- unless something like a crash or pothole prematurely finishes off a rim, spokes generally fatigue first. It might make sense to reuse spokes from a wheel that didn't see much life, but it definitely doesn't make sense to reuse spokes nearing the end of their life cycle.

3- as far as ERD's are concerned, 2mm shouldn't be make or break since it's only 1mm difference in spoke/nipple engagement. (difference between diameter and radius). But if for example the original spokes were coming to the tops of the nipples, another millimeter higher could be a deal breaker.

I also don't trust published ERD, and do all my spoke calculations based on my own measurements. Part of the issue is that some people, inclucing myself measure rims to the nipple seat (actual measurement). Others measure the ERD to the desired spoke height which is 2-3mm per side longer to account spoke reaching into the nipple's head. (expect mrrabbit to chime in here). IMO either way is OK as long as you use the data accordingly, but you need to know what you have, and that isn't always clear. There's 4-5mm difference in the two measurements, which is plenty enough to make any calculation useless (GIGO).