Just remember that there may be changes to a lot of tax laws before January.
Do you have the 2016 desktop software installed? If so, you have a couple of options for experimenting.
Before experimenting, be sure to save an archival copy of your 2016 data file as originally filed; i.e., the *.tax2016 data file. Name it something like "2016return_original" or whatever you choose, and store it in a safe place. It's also a good idea to save some PDFs. I usually save a PDF of my return "as filed", and one with all the forms and worksheets.
If you have the 2016 software, you can prepare multiple returns, so just start a new one. Be sure to save it with a completely different filename from your original 2016 return. Such as "2017_whatif" or "2017test", etc.
If you don't want to start from scratch and plug in everything, there is a "what-if" feature built into the desktop software that uses your 2016 return as a foundation. If you decide to use that, then before doing it, I'd make an archival copy of my 2016 tax data file and PDFs and store them safely as I mentioned above. The "what-if" feature should not be a permanent change since it's a separate worksheet, but if something goes wrong, it's good to have the original data file archived as a backup.
To get to the "what-if" feature, open your 2016 return, then switch to Forms Mode. Above the list of forms in the left column is an "Open Form" icon. Click that, then in the search field enter "what" without quotes. Then select the "What-If Worksheet."
