Can I copy an existing return and do a 'what if' without amending the return? I want to see how an IRA rollover to a ROTH will affect my return for next year.

Only if you are using the Desktop installed program.  The Desktop program has the What-If worksheet and you can save your return with another name to test in.

But not in the Online version.  Do not change your Online return or you will overwrite it!

You can download an online .tax2018 file and open it in the Desktop program.  Or you can setup a new online account and do another return.  You don't have to pay unless you want to print it.  Online is only good for 1 return.  But you can use the same email address for 5 accounts.   How to start another return in the Online version https://ttlc.intuit.com/replies/3288574 

This tool may be useful in preparing test return results:

https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1