Yes, you will need to re-open your return, then make the change.
- In TurboTax Online, sign into your account.
- Select Tax Home in the left menu.
- Scroll down to Your tax returns & documents.
- 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
