How do we set OS X Lion sync to show warning before data on computer is changed?
If you still have your snapshot backup of your OSX Snow Leopard before you upgraded to Lion (you did backup didn't you?), then there is a solution to getting iSync back up and working under Lion :

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Extract follows :


  Before installation of Lion, I had taken a full disk image of my
  previous Snow Leopard installation using the excellent SuperDuper!
  Disk cloner.
  
  This was more from a backup and recovery perspective, but allowed me
  an unexpected solution to the iSync quandary – on a whim, I attached
  the Snow Leopard disk to the Mac running Lion, and clicked on the
  iSync application. Voila! iSync works exactly as it should!


Hope this helps.