The online software only prepares your taxes for the 2019 tax return. Any input from your 2018 account will not impact the preparation of your 2019 taxes.
The only issue you would have is updating your 2018 Adjusted Gross Income amount when you get to the File section of the program and potentially any carryover amounts, if applicable.
Otherwise, you can go ahead and prepare your 2019 tax return even though the 2018 amounts show up in the prior year column.
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There is a tricky way to clear out transferred info in the Online version. But doing this will also clear out all your entries. Go to Tax Tools > Tools > Transfer from your computer > Continue > then Continue without transferring.
Or you can just start a brand new account with a new username. How to start another return in the Online version