Can I try TurboTax Deluxe but then downgrade back to Basic if I don't qualify for any of the deductions or if my itemized deductions are less than the standard deduction?

Yes, clear and start over is the only way to be certian that incorrect information does not linger. Some entries trigger other things that are not always noticable.

The other option is to downgrade to the Free Edition. Then you could upgrade back to Deluxe as you go. 

But as leeloo suggested, some things may get wiped, or linger going from one to the other.

Can you add up your deductions and compare to your standard deduction for your filing status instead? @Jordan40 

It would help determine whether it's worth the upgrade/ start over hassle, but you do make a good point.  

But TurboTax - as you have already determined - does not undo versions.

@Jordan40