Will this email my existing subscribers?
During the initial sync, the connector will pause any automations that are tied to the audience you connected. In addition to that, all e-commerce based automations that are tied to the connected shop will be paused while the sync is running. If you have the connector setting enabled to subscribe Shopify customers that don't exist in Mailchimp, we recommend that you remove subscribers tagged with "subscribed-from-shopify" from any welcome automation you plan to re-enable. You may need to activate advanced settings on the automation to enable you to remove subscribers from the queue. This should stop those automations from emailing subscribers that may have been subscribed during the initial sync.
Please note, if you have a double opt-in enabled audience and you have the connector setting enabled to subscribe Shopify customers they will be sent a confirmation email from Mailchimp before they are considered subscribers in Mailchimp.