will Apple allow me to trade in a CDMA iphone 5 for a GSM iphone 5?

You can only return it to the Apple Store if it was originally purchased there. If you are within the timeframe for the Apple Store return policy, you can return/exchange with no questions asked. In the US, the policy is within 30-days if your iPhone is contracted to a carrier or 14 days if your iPhone is unlocked. With a carrier-locked iPhone plane to spend some anount of time dealing with the carrier to get your upgrade status reset.

Think of an auto mechanic's service bay. You tow in a a disabled car to have it fixed. When the repair is done, the mechanic hands you the keys to a different model with a different engine and different option package. Great deal right? Unfortunately, real life does not work this way.

If you want a different model, your solution is not in the service department, but on the sales floor. The Apple store is no different.

You dont have to trust what I say, although in this case, you should.

APple provides all hardware support in the US and most other ccountries.

WHat you describe above is a warranty replacement of a device purchased from Amazon. This is normal.

THis is not what you are asking for.

AS already stated, Apple does not provide a trade-in program. If the device is outside the return window, well it and buy the model you desire.