From Contact Form 7

While Contact Form 7 forms exist on the site, Contact Form → Settings has a Migrate from CF7 tab.

1. Open Settings → Migrate from CF7. It lists each Contact Form 7 form. The In Re:Maily column links to the copy of any form you imported before, or says Not imported yet.
2. Tick the forms to bring over, or Select all, and click Import selected forms. New Re:Maily forms are made; nothing is overwritten.
3. Open each new form in Re:Maily’s editor and check it. Preview shows it with your theme’s styles.

Each form comes over with its fields and their options, both mails, its messages and its additional settings (demo mode, skip mail, subscribers only). Field names stay the same, so mail tags keep working. Your Contact Form 7 forms aren’t changed, and importing a form again makes another copy.

Your existing pages

Once you turn Contact Form 7 off, its [contact-form-7 id="…"] shortcodes show the imported Re:Maily forms, so pages keep working while you replace the shortcodes at your own pace.