DA2000 First Officer describes switching to autopilot #2 at FL430 when ADC #1 fails. Unfortunately the autopilot reverts to pitch and roll when during the switchover and an altitude deviation of 500 FT develops before it is detected. Flight diverts for maintenance.

Narrative

During cruise we encountered a #1 ADC failure. After reviewing the checklist and talking to company maintenance the Captain switched the autopilot over to the #2 ADC Copilot side and THEN informed me that I had the controls. Unfortunately; during the switch to the #2 side the autopilot reverts to roll and pitch; between the time of the switch and being informed of the transfer of control; the aircraft had climbed over 400 but less then 500 FT above FL430. After noticing the discrepancy; I engaged the heading and pitched back down to FL430. I notified ATC of the discrepancy and that I was returning to the assigned altitude. We never declared an emergency; however we had notified all centers of the abnormality and eventual failure of the ADC. We checked our altitude readouts against theirs; everything on the #2 side was accurate so I believe that we may have had an altitude deviation. Also; after we did the transfer; we diverted. I notified the Center at that time that we were no longer RVSM compliant and that we could start down at any time. We initiated a slow descent thereafter per ATC. The flight finished without any other complications.