Its my understanding that fat adaptation is the cellular up regulation of certain transporters that facilitate the oxidation of fatty acids. This process takes time to develop and is not quickly reversed.
A day of increased carb intake may temporarily increase weight, raise blood glucose, halt ketosis, etc. But by whatever metric (ie. weight, insulin sensitivity, body fat) you choose to measure, the progress that’s been made over a 6 week period cannot be undone in a day.
The only caveat is food addiction, which I am unqualified to speak about.