Except when income must be reported on the apportionment basis, permission to make a change either from separate accounting to apportionment, or vice versa shall be obtained in writing from the department upon written application setting forth in detail the reasons why the desired change will more clearly reflect the taxpayer's Wisconsin income. Such application shall be mailed to the Wisconsin Department of Revenue, P.O. Box 8906, Madison, WI 53708-8906 before the end of the tax year for which the change is desired.
Wis. Admin. Code Department of Revenue Tax 2.44
Section Tax 2.44 interpretss. 71.04(4) and 71.25(6), Stats.