Challenging the rules: https://www.pbs.org/newshour/politics/supreme-court-revives-...
Changing the rules at USPS: https://www.pbs.org/newshour/nation/how-this-new-mail-rule-c...
And I'd fully expect some fuckery via executive orders closer to the election, and SCOTUS to use the emergency docket to let them "temporarily" be enforced.