When you enable the replace existing subscribers option, a new setting called replace interest groupings appears.
Here we will clarify what this setting actually does.
Replace Interest Groupings
When enabling the "update existing subscribers" setting it means that people may already be subscribed to the selected Mailchimp list(s) when using that form or integration.
In case someone is already subscribed to a list, their profile in Mailchimp will be updated with the new data the plugin sends to Mailchimp. With the "replace interest groupings" setting enabled, any previously selected interests by this subscriber will be overwritten.
Disabling the setting will add the selected interests to any previously selected interests.
John previously subscribed and selected the "Fruits" grouping.
A few months later, John fills out the form again but he selects (only) the "Vegetables" interest this time.
With the replace interests groupings setting enabled, John will now only be part of the "Vegetables" interest, as opposed to both "Fruits" and "Vegetables".