Tesla's connector is no more proprietary than any of the other connectors out there. In fact it's less proprietary as they openly and freely license their standard. As they have by far the largest and most capable charge network in the US (as well as most everywhere else), it really would make little sense for the US government to mandate any other connector standard. IMO, the only connector that makes just as much sense, would be the same connector already adopted by the EU. In an attempt for a global standard. If they pick any other connector, it means corporate interests played too big a role in dictating the newly decided standard.
So if it does happen and it becomes a plug other than what Tesla is currently using, I would assume new cars would simply come with the new plug. Existing vehicles would use an adapter. For vehicles that are currently in production where they can offer correct parts, we'd have the option to retrofit the new charge port. We may see a time where new cars include both ports to help ease the transition.