Changing an existing provider to a custom provider

Hello all,
    I have a website that has been running for a couple of years using membership for its page restriction process. Well I was just informed that the client does not like the password requirements. I need to be able to change the membership provider in the web.config file while not changing anything for the currently registered users. Is this possible? Or am I worrying about something unneccessarily?
 
Thanks 
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