If you remove a team member with a time estimate, you will be have the option to add their time to another team member. This is to ensure that your total work estimate does not change.
If a work team member's tasks, emails or notes related to the work are reassigned to another colleague, they will remain part of the work team and any time estimated for them will remain. If you want to move the time to another colleague too, you are required to do this manually. Once again, this is to ensure the total work estimate does not change.