I found a question that popped up a few times on discussions blogs:

Question:
I incorrectly configured the SUP on one of my remote sites and so all these clients are looking at the parent site for Software Updates. I've checked the WUAHandler.log and have seen the settings there. Now that I've got things correctly configured, I'm wondering how I can 'force' the clients to re-home to their local SUP instead. Do I need to reinstall the clients to apply the SCCM local policies for this?

Answer:
No you do not need to reinstall the clients. If clients are able to connect to the assigned MP, clients will automatically pick up the new WSUS URL and use it. Default ConfigMgr Site Policy interval is one hour. So after one hour your clients should re-home and use the new WSUS URL.