Setting aside the question of whether Congress can alter the matter short of a constitutional amendment, doing so using something like federal legislation, if the bill would actually apply to the incidents of Jan 6, I have a hard time seeing how that wouldn’t amount to being an ex post facto law.
An ex post facto law is a law that retroactively changes the legal consequences (or status) of actions that were committed, or relationships that existed, before the enactment of the law. In criminal law, it may criminalize actions that were legal when committed; it may aggravate a crime by bringing it into a more severe category than it was in when it was committed; it may change the punishment prescribed for a crime, as by adding new penalties or extending sentences; it may extend the statute of limitations; or it may alter the rules of evidence in order to make conviction for a crime likelier than it would have been when the deed was committed.
Ex post facto laws are expressly forbidden by the United States Constitution in Article 1, Section 9, Clause 3 (with respect to federal laws) and Article 1, Section 10 (with respect to state laws).