I had the same problem with my modular alert action and the best solution I could find so far is to add a second block for changing the existing stored credentials:
<block title="Change credentials" endpoint="storage/passwords" entity="*" mode="iter">
<input field="password">
<label>Password for $name$</label>
<type>password</type>
</input>
</block>
See also: https://docs.splunk.com/Documentation/Splunk/latest/Admin/Setup.xmlconf
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