What happens to existing mags all depends on grandfathering. From what they've proposed my understanding is mags out there currently that you own would be fine. Depending on how strict they go you might not be able to sell them, however. I have 6 magazines for all my pistols, I'm not worried. Even if I had only 10 rds, I'd still go with 9mm. To me if you honestly would change what caliber you carry simply because you have 10 instead of say 15 rds, maybe you shouldn't have been carrying that caliber in the first place as you don't seem to have a ton of confidence in it.
Guns don't kill people. Apes with guns kill people! - Robin Williams