It depends also a bit what you mean with 'existing'. Do you want to keep using the Volto Frontend and the Plone html 'server side' frontend at the same time, existing next to each other as Timo interprets the question?
Or do you want to 'move' the existing site to using the Volto frontend exclusively? Those are two different paths.
As Timo explains the main difference is with the 'rich text' fields (like the body text on the Document CT', which has a different storage structure. If you migrate the existing HTML snippets to the blocks field, you are halfway there. At the PloneConf 2019 in Ferrara a migration solution was presented for these fields, but it's not trivial because you have to run javascript and Python side by side. The RedTurtle team has some experience with this AFAIK and has created some packages. (i.e. GitHub - RedTurtle/redturtle.importer.volto: Used to import an old Plone Site into a new with Volto)
The second challenge is that Volto on a new Plone site uses a variant of the normal document type which is a folderish page. Default Plone setups uses separate content types folder and page. If you migrate an existing site to Volto you will also have to look at this difference and for example migrate in a folder its 'item as default view' Document Richtext Field to the parent Folderish Page Block field.
I'm considering such a migation myself for the end of this year for a larget site, this is just the general overview of the main challenges as I see them for that project, other community members might have experience and more detailed information.