did your prior accountants fully depreciate the assets by taking either section 179 and/or bonus/special depreciation? this would have resulted in expensing (fully depreciating) the entire cost in the year they were put into service. if so, the wrong lives would not matter because there is nothing left to depreciate.
if they didn't you can not just change their lives. technically you would be required to file form 3115 to correct these errors. this is probably something that you would need a pro to do so it's done correctly. this would be the proper thing to do. however. if not a lot of $$$'s are involved I would think twice about going that route and just leave things as they are.
