I don't agree with @JohnB5677 response regarding amending prior returns.
Essentially what has happened is you have an incorrect method of accounting for those items that you did not depreciate. As a result, you will need to file Form 3115 to correct this and claim any depreciation not taken in those years. When completing Form 3115, you will compute a Section 481(a) adjustment and this will be used in the current year tax return. This can be fairly complicated, so I would recommend meeting with a tax professional to help you in this area.
Generally you need to request consent from the IRS to change from an incorrect method to a correct method, however, there are several areas where the consent is automatic. A change for missed depreciation is one of those areas. However, you need to be able to cite the correct authority when filing the Form 3115 and a tax professional will be able to help you there.
*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.
