Any IRS code section or tax law case supporting the following?

Scenario:

Amend to change from Actual to Standard Mileage method in first year?

2015 tax return was filed and accepted:
First year (2015) vehicle placed in service with the “ACTUAL METHOD” by Preparer A when STM (standard mileage method) should have been selected.

2016 tax return: Not filed - on extension
Different Preparer B is looking to determine if the method selected in 2015 can be amended before 2016 tax return is filed or if the original method is irrevocable