I don't have a conclusion as to whether the item in bold applies to your situation. The 15-year life for QRP was under code section 168(e)(7) which is no longer in the code. Others may offer their opinion. However, you may want to play it safe and consult a tax pro. If AMC does apply that would require filing form 3115.
If ACM does not apply you can amend the open years but not for depreciation that should have been taken before that. That isn't recovered until the property is disposed of.
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Regs. Sec. 1.446-1(e)(2)(ii)(d)(3)(i) retains the rule from the temporary regulations that an accounting-method change (AMC) does not include an adjustment in the useful life of a depreciable or amortizable asset for which depreciation is determined, unless the taxpayer is changing to or from a useful life (or recovery or amortization period) specifically assigned by the Code, the regulations or other guidance published in the Internal Revenue Bulletin.
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(3) Changes in depreciation or amortization that are not a change in method of accounting. Section 1.446-1(e)(2)(ii)(b) applies to determine whether a change in depreciation or amortization is not a change in method of accounting. Further, the following changes in depreciation or amortization are not a change in method of accounting:
(i) Useful life. An adjustment in the useful life of a depreciable or amortizable asset for which depreciation is determined under section 167 (other than under section 168, section 1400I, section 1400L(c), former section 168, or an additional first year depreciation deduction provision of the Internal Revenue Code (for example, section 168(k), 1400L(b), or 1400N(d))) is not a change in method of accounting. This paragraph (e)(2)(ii)(d)(3)(i) does not apply if a taxpayer is changing to or from a useful life (or recovery period or amortization period) that is specifically assigned by the Internal Revenue Code (for example, section 167(f)(1), section 168(c), section 168(g)(2) or (3), section 197), the regulations under the Internal Revenue Code, or other guidance published in the Internal Revenue Bulletin and, therefore, such change is a change in method of accounting.
