If you want to use your 2018 information to create the what-if scenario, you can create a new account using TurboTax Online.  Enter 'dummy' information into the return and just do not follow-through with filing the return.  You can clear it when you are finished testing.  There would be no charges incurred to do this.

Or, if you are using a CD/downloaded version of TurboTax, you can use the File > Save As function to create a dummy copy of the return you just filed and then edit that copy to create the what-if scenario. 

Using your 2018 information will give you some idea of how it would affect your return overall if all of the income were reported on 1099-MISC.