Ok if the 05 is now yours legally you should be able to file for mechanics lean, abandoned car . But basically you take vin and proof of ownership to dealer and they can current key. BUT if your up to it simply swapping wiring, bcm/dash/cluster/HVAC all from 05 to 04. You would pretty much turn car electronically/mechanically into an 05, would possibly issue for dealership services. BUT you could also pick up Chinese knock off tech 2 or GM GDI to do stock services. If you ever sold it, well might be little iffy. As for learning a key, call mobile locksmith most can cut and clear/program on keys easy enough. Also if you can get key cut to program it all you do is hook battery charger up, insert key, try to start it leave on run setting for 10 min till security light quits blinking turn it off, turn back on but not try to start let sit for 10 minutes, repeat on more time and it should accept new transponder key. Just Google passkey 3 relearn.
