There is always "transitional authority" by virtue of the writing of regulations. The statute generally allows the president (via the secretary of HHS) to draft regulations which present the administration's interpretation of the statute that was enacted. Often, that authority is fairly broad.
The regulations under the ACA have been overly broad and the courts will have to sort this out if a future president doesn't fix the problem. The statute is generally the authoritative source, however, it is possible for courts to rely on the legislative history to discern the underlying intent of the statute.
Of greater concern is the administration's blatant disregard of the statue, for example, the recent decision to divert funds from other sources to reimburse insurance companies for their losses. This is an outright violation of the law as the law was specific on the subject of how the "risk corridor" provision was to be determined. The administration can't just change that (particularly, for political purposes, as has been done).
Unfortunately, on many of these items, they will be over and done with long before the courts act on them; so even though the president is breaking the law, nothing will be done about it.
The bigger problem is that the law is totally unconstitutional, because it raises billions in revenue for the general fund and the legislation didn't originate in the House, which the Constitution requires for good reason. The expectation is that the Court will not move against Congress on this, but that presumption may or may not be right. We could well see the ENTIRE law thrown out for this reason alone.