Can I change from Standard to Itemized deductions after choosing File by Mail but not mailed yet?

[KrisD]

Yes, you will need to re-open your return, then make the change. 

1. In TurboTax Online, sign into your account.
2. Select Tax Home in the left menu.
3. Scroll down to Your tax returns & documents.
4. Select 2018, and then select Add a State (you're not actually adding a state, this just gets you into the right menu).

Now change to Itemized: 

https://ttlc.intuit.com/replies/3300919