There is no rule/regulation. However, it falls under what I call "A Very Bad Idea." Here is why:
1. Patient has Aetna. You bill $200 for a service. You expect Aetna will pay $150. Aetna denies, because the insurance terminated. Patient says, oh yeah, I have United now. You transfer the bill to United and send it out. United pays you $200. What does this mean? Most likely, United would have paid more; you didn't even bill as much as the allowed amount and your providers are losing money they are entitled to. If there is no write off, you didn't bill enough.
2. Patient has Aetna. You bill $200 for a service. You expect Aetna will pay $150. Aetna denies, because the insurance terminated. Patient says, oh yeah, I have Medicare now. You transfer the bill to Medicare and send it out. Medicare allows $200, same reason as above, and pays $160 (80%), with a $40 patient balance. Except Medicare has it on record that you usually bill them $225 for that same service, and you're now in a swirling black hole of what I call "Audit Land." Medicare says you are billing them more than your usual and customary rate, which you've now told them is $200, not $225. The problem gets bigger:
When you bill $225 like you did every other time, Medicare allows $220 (the actual fee schedule) and pays you $176, leaving $44 patient balance. Now the fun begins at Audit Land. Medicare wants $16 back for every time you billed Medicare $225 instead of $200 for the CPT code. Now multiply that by how many Medicare patients you have, and multiply THAT by how many Medicare claims you have spread all across all the CPT codes. I'm not sure if Medicare is still doing what is essentially an estimated overbilling amount (because I've stopped every provider I've ever worked with or consulted for from doing this). This means, if they sample 100 claims, and they show you were overpaid $2000 on those 100 claims, they multiply $20 ($2000 divided by 100) times the total number of Medicare claims for a certain number of years, and that's how much you have to pay back, whether or not you were actually overpaid that amount.
Always bill the same amount for the same CPT code for any payer. Stay out of Audit Land. It is not a fun place.
Bonus: If you do legal work, you usually have to provide your fee schedule to both sides of the case. They will always ask you if this is your usual and customary rates (and if you are called to testify about billing, you will have to answer this under oath). Then they will say that you are dishonest if you are billing their client an amount that is not the lowest amount that you bill anyone.
