Unless you're having some issues with your budget, I'd do it while the car is apart. The rebuilder (name escapes me at the moment, but it's in my thread) doesn't sell new ones, only rebuilds your cores. So that means you have to take your car off the road, make it immobile, and wait for the parts to get there, be rebuilt, and get back. Mine took something like a week. Depending on his work load, it may take considerably longer. Add to that the shipping to and from Germany, and it would be a major hassle.
Now, however, you could wrap them up, get them to my friend when he travels to Bad Durkheim this winter, and get them back before the car gets back together. 6 months of transit time doesn't affect the rebuild time on the car, I'd imagine.
Just my two phennigs.