Since we've moved to another flat, I've been obliged to change the address stated on my car registration form (carte de grise). The rules state that you have one month to make the change after you have moved.
I've been putting it off, cause anyone who've tried to deal with French administration will agree with me, it is a royal pain in the ass. You need 40 forms completely correctly filled out, and each of those you need in another 40 copies etc etc..
This time we had cheated a bit, and printed out the form of their website (They actually have a website.. wow) - so when I finally went there I had all needed papers and copies.
Normally when I go there (I've been there a few times) I wait over 2 hours.. but this time I actually managed to get it all done in less than 40 minutes !! Perhaps it's because everyone is on vacation or something, anyway I'm happy as hell that it all went that painless.
To conclude.. go at the prefecture in august.. or don't move - I don't really care.