Your forum data is stored in the database, the files are what process the data and make it appear on your screen. Overwriting the files won't affect the actual data stored in the database (until you run the upgrade script).
You will need to revert custom templates, a message should appear in your AdminCP to highlight out of date templates. You will need to revert the templates (losing any customisations) or copy across changed code (can be confusing and time consuming). Most modifications should remain intact, however some make break (especially due to the security token update in 3.6.10 & 3.7).
Remember to take a back-up of your database before you make any major changes.
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