If you have admin access on your CommCareHQ CommCare HQ project space, you can transfer/re-assign cases that are registered on one Mobile Workers case list to another Mobile Worker through the Data tab available at the top of CommCare HQ.
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When you transfer cases that belong to mobile user "Lalita" to mobile user "Sheela", the transferred cases will be reflected on Sheela's case list on her phone once her phone syncs to CommCareHQCommCare HQ. On the server side, form submissions made by Lalita will always be linked to username Lalita because she completed those forms. The change will however be reflected in the case exports, which will show Sheela as the new owner of the case. Any new form submissions completed by Sheela for the transferred case will reflect Sheela as the username in the forms exports. The old submissions for that case will still remain under the username "Lalita".