Can I use my existing 2016 return (not yet filed) to project different income amounts and sources?

[rjs]

You indicated that you are using the CD or download TurboTax software, so you can use Save As to save a copy of your tax return with a different name. Then you can make changes to the copy to see the effect, without affecting your actual tax return. If you want to save different scenarios, you can save multiple copies with different names. Be careful to keep track of the names you use, or make them very descriptive, so you don't get confused about which file is your actual return. Also, when you start your 2017 tax return next year, make sure you select the actual return to transfer information from.

Another option is to use the What-If Worksheet in forms mode. It lets you use your existing return as a starting point and create different scenarios without affecting the actual tax return.