You may encounter problems attempting to edit a time entry if it involves an activity that is assigned to a specific project. For example, a project manager might be trying to edit a time entry and is unable to change the project.
If the time entry is for an activity assigned to a specific project, you have to assign that activity to this new project first to be able to bill it to that project. You can do this by opening Projects > Manage Projects from the side menu. Select the project you want to want to assign the activity item to and go to the General > Assignment tab. Choose to Show > Activity Items and click Add Activity Item. Select the appropriate Activity Item from the list or you can even add a Group of Activity Items, and then click Add. Now that the activity is assigned to the new project, time entries for this specific activity can be billed to it. Check CORE Help Center for details.