Cash to accrual

Check IRS Pub 538.  Also the instructions for Form 3115.

Change in Accounting Method
Generally, you can choose any permitted accounting method when you file your first tax return. You do not need to obtain IRS approval to choose the initial accounting method. You must, however, use the method consistently from year to year and it must clearly reflect your income..
Once you have set up your accounting method and filed your first return, generally, you must receive approval from the IRS before you change the method. In general, you must file a current Form 3115 to request a change in either an overall accounting method or the accounting treatment of any item.
A change in your accounting method includes a change not only in your overall system of accounting but also in the treatment of any material item. A material item is one that affects the proper time for inclusion of income or allowance of a deduction. Although an accounting method can exist without treating an item consistently, an accounting method is not established for that item, in most cases, unless the item is treated consistently.
Approval required. The following are examples of changes in accounting method that require IRS approval.
• A change from the cash method to an accrual method or vice versa.
• A change in the method or basis used to value inventory.
• A change in the depreciation or amortization method (except for certain permitted changes to the straight-line method).
Approval not required. The following are examples of types of changes that are not changes in accounting methods and do not require IRS approval.
• Correction of a math or posting error.
• Correction of an error in figuring tax liability (such as an error in figuring a credit).
• An adjustment of any item of income or deduction that does not involve the proper time for including it in income or deducting it.
• Certain adjustments in the useful life of a depreciable or amortizable asset.
For additional information, see the Instructions for Form 3115.