On some platforms (e.g. Gentoo) ROS 1 users are already using Python 3 today. So “some part” of the code base is already Python 3 compatible. The amount of work necessary is hard to estimate. From my experience changing existing Python code to support both (version 2 and version 3) is commonly fairly simple. There are only very few cases where it is more difficult. As soon as a distro would be setup offering Jenkins bases CI / PR jobs using Python 3 those should be fixable iteratively. This is also a task where contributions from the community could go a long way to spread the workload.