I agree with @tagteam in that amending for the original OP (original poster) is not permissible. The reason for this is that the tax code states that if you have filed a tax return for two or more years, you have established a method of accounting. Doesn't matter whether the method is right or wrong, a method has been established.
The only way to correct this is to file a form 3115 as has been discussed. This would include ALL years. You can't file a form 3115 for part and amend part. Just not the way it works.
So bottom line, if you have filed a tax return for more than one year (two years) and have not claimed depreciation, then you MUST file a form 3115 to change the method of accounting. The form 3115 will determine a 481(a) adjustment that will provide a catch-up on any missed depreciation.
As has been stated above, this is not an easy task and professional assistance is recommended.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
