IRC section 446(e) states that, unless otherwise provided, a taxpayer, whether an individual or a business, must secure the IRS's consent before changing a method of accounting for any item for federal income tax purposes. Form 3115 is used to request this consent.
There are two types of consent - automatic and non-automatic. Automatic consent is granted for changes that have an assigned Designated Change Number (DCN). The form instructions provides a list of possible DCNs. When requesting automatic consent with a DCN, Form 3115 is attached to the annual tax return and a signed copy of the form is mailed to the IRS.
A taxpayer applying for a non-automatic change, however, needs to file Form 3115 during the tax year to which the change applies and as early as possible to give the IRS enough time to respond. There is a fee for a non-automatic change (ranging from $2,800 to $10,800 as of February, 2019).
Before filing Form 3115, read its instructions as well as the other relevant IRS form instructions and publications to determine if the ability to make the change without filing the form is already provided for in the code.