Without knowing the details of exactly what this hospital is doing, it's hard to comment on whether or not it is appropriate. But in my experience, this is a common practice with hospitals, and though I'm not an expert on the regulations here, I don't see a compliance issue with doing this. The reason I say that is that the root cause of the denials in such cases is with the insurance companies disagreeing with the physician's assessment that the patient should have been classified as an inpatient for treatment purposes. They are typically not stating that the entire service was medically unnecessary or should not have been covered at all, but only that the patient was inappropriately classified as inpatient when the services could have been performed as outpatient.
If the payer is not willing to yield on this point, then I feel that it is entirely appropriate to submit the claim as outpatient - it is not fraudulent to do this because the services themselves are still supported by documentation and reported correctly - it's just that the claim is being submitted according to the outpatient classification that the payer has determined was appropriate for the care that was rendered. Also, in cases like this, the payers have already reviewed the hospital records and are fully aware of what is in the documentation and that the hospitals will be resubmitting these claims as outpatient bills - nothing is being done surreptitiously or with an intent to deceive the payer. And if the services were truly not covered, then the outpatient would be denied as well.