You can run both at the same time. Any software that is not IPv6 compatible would still be able to connect through IPv4 and just not have access to IPv6 addresses.
The bigger issue would be connecting to IPv6 only servers from an IPv4 program. The first question is how many IPv4 only programs exist in the wild. If you happen to have one, and cannot update it, then you still have the option of creating an IPv4 to IPv6 proxy for the specific site(s) you need.
Comments
You can run both at the same time. Any software that is not IPv6 compatible would still be able to connect through IPv4 and just not have access to IPv6 addresses.
The bigger issue would be connecting to IPv6 only servers from an IPv4 program. The first question is how many IPv4 only programs exist in the wild. If you happen to have one, and cannot update it, then you still have the option of creating an IPv4 to IPv6 proxy for the specific site(s) you need.