Quote from Ghune
Thanks for the replies, guys. It's good, I have free space (lots of chunks) around my "city". I know that restarting a new world would be much better but after all, I just have to walk for 2 or 3 days in a same direction and it will be like a new world. I will just connect them with a long underground railroad.
I really suggest you start a new world. You'll get really terrible looking terrain generation from 1.7.3 chunks to 1.8 chunks. It doesn't matter how much you have built, it's not worth it.