Hi Raymond,
When you move a user story (and its tasks), the estimates and to dos do not change. So, just as you mention, your team members will appear to be over-allocated based on those estimates. What I would do is make sure to apply reasonable estimates on those tasks based on their
current state, not the state when the tasks were created. Does that make sense? Basically, estimate based on the amount of work already done.
There's actually a great Rally Help article on this at:
https://help.rallydev.com/manage-unfinished-work