The major problem that you have is that the fields will have the same names but not the same field keys. Otherwise you use the standard WP export and import. This will be a problem exporting an importing with almost any tool. You’d need to look up the fields to get the field keys to insert.
WP All Import is not a free plugin, but the pro version has an addon that will import into ACF fields. That’s probably the path I’d take unless I had a lot of time to kill that I could spend building something to do it myself http://www.wpallimport.com/
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