If I get an IPv6 from my ISP for my new office branch, will I be able to communicate with my old branch that is using IPv4?
Provided that you get both, then yes. You need IPv4 to communicate with IPv4 hosts and IPv6 to communicate with IPv6 hosts.
Technically, there is "NAT-PT", NAT Protocol Translation, to let you access IPv4 stuff when you only have IPv6, but that would need to be run by the ISP and I wouldn't want to try to set up a VPN through that.
So adding IPv6 won't break IPv4 stuff. Having only IPv6 would be a problem.
You should be fine (there's not a lot IPv6 around yet), but it depends on the implementation in your hardware and software:
- On the operating system side: No problem, all modern operating systems are dual stack, i.e. they support both IPv4 and IPv6 and choose the proper version automatically.
- You'll probably get new hardware, i.e. an IPv6 router. It's reasonable to assume that it will be dual stack, but you might need to configure it properly.
If all else fails, you can still fall back to a 6to4 tunnel which practically wraps your IPv6 packets in IPv4 packets.