So, you have 2 phones.  The original iPhone and, let’s say a Motorola RAZR. You want to upgrade your RAZR to the new iPhone 3G but it’s not eligible to upgrade.  Here’s what you do.

Since the original iPhone is automatically eligible to upgrade to the new iPhone, go ahead an upgrade that device.  once it is active you may then take the SIM card from your RAZR and put it in the 3g iPhone.   Re-Sync to iTunes and bingo bango, your RAZR line is now anew iPhone.

Now, keep in mind that is best case scenario, but we all know they track record of AT&T and Apple so expect at least some complications.  You may need to call customer service to complete the process but it can be done.