There is no need to test or change or duplicate anything, QBDT and even all of QBO is  by nature and default, ACCRUAL, with the option to view reports as CASH.This is so, despite what you might choose or have chosen in your cpany settings when you started with QB.

Simply run reports and make sure you select Accrual rather than Cash  or run a set of both to compare.

The IRS allows you to change reporting but unless you plan on issuing half year forms (which I guarantee they won't understand) ypu should only change from one type to another at fiscal year rollover. 

There is a limit with the IRS on how often you can change back and forth - you cannot change just because one set of numbers in a certain year results in lower taxes.

So of you are satisfied with accrual reports from your existing company file just edit the company settings to change to accrual. That itself does not change a dime in transactions. But it will check the box for 2021 forms that you ate now accrual