If it's just a coding error, you shouldn't need to waste an appeal. Ask them what the prior code was & what the new code is. Sometimes you can find the codes online -- if it's covered by Medicare you are very, very likely to eventually win an appeal/revision of coding. If you can't find the code online (and maybe it's a good idea even if you can), call your insurance company and ask them whether that code is typically covered. They won't abide by what they tell you, but write down their statement, the date/time of the call & the name of the rep anyways. If you have to appeal to the Div. of Insurance that is something you can include if the rep tells you it will be covered.
If they say it won't be covered, ask if they can provide any guidance on what the correct code might be (it's worked for me a little less than half the time). If they can't do that, ask them if they know why it's not being covered and if there's any additional information they can offer. Depending on your state, your doctors office has a set period of time to try billing under different codes (in IL it's 6 months; your insurer should be able to tell you what it is in your state). Don't miss that deadline. If you (read: your doctor) can't get the billing code correct by that deadline, your appeal rights may be affected. All hope isn't lost, but it makes it harder.
In the future, you might try asking for a Pre-Determination Letter. NOT a Pre-Approval Letter (those things are practically worthless). Your doctor needs to request it, but you can push his office to do so. Yes, it's a huge hassle & the insurance company will complain bitterly, but what other option have they left us? I've been screwed over so many times with those Pre-Approval Letters that it's ridiculous. I've learned to fight back better now, but it cost me a lot of money to learn that lesson.