At the moment, when moving items from one board to another that contain subitems, the subitems will not be able to transfer over and they will be deleted.
With this in mind, our developers are working hard on adding and enabling upcoming features as soon as possible, but to be fully transparent they will be coming in increments as we want to ensure that each functionality is released with the best quality possible.
As always, you can keep up with the roadmap for subitems here - however, please note that while this information is shared to outline some of our plans, these plans are subject to change.