Historically, Dassault did not offer a direct "downgrader." Their official stance was: "Everyone must be on the same version." However, user pressure has forced a shift. The is a standalone utility (available to active subscription customers) that strips the "future" tagging and allows a read-only or converted backward save.
In recent years, Dassault Systèmes added a specific feature to the free (accessible via the web) to "Unsuppress" geometry, effectively acting as a converter. solidworks future version file converter link