The Congressional Review Act (CRA) allows Congress to undo certain rules not officially submitted with a simple majority. This includes some ACA related guidance.
This loophole of sorts won’t necessarily be used, and even if it is there are only a few measures it can be applied to, but it is still worth noting.
Essentially precedent was set this week when the Senate used this method to repeal regulatory guidance on the auto industry issued by the Consumer Financial Protection Bureau (CFPB) in 2013. This can’t be applied to rules that have been officially submitted (in the way that the CRA deems official; in short it needs to be submitted for a review and published in the Federal Register), but it can be applied to guidance that didn’t go through the the required process.
One example of what this could apply to was a set of guidance providing guidance on section 1332 waivers (which allow states to set up unique state-based ACA alternatives).
That is the simple version of a somewhat complicated issue, you can get a more robust version of the story from The Federalist’s New Precedent Allows Congress to Dismantle (Some Of) Obamacare.