I mistakenly set up my nonprofit as an LLC for a year but plan to convert it to a corporation this year. What tax return should I file for last year's nonprofit LLC?

did you request tax exemption from the iRS?  Some set up what they think are non-profits but don't get IRS approval and are surprised when The IRS gives them a bill. 

see this webpage for guidance

https://www.forbes.com/advisor/business/how-to-start-a-nonprofit/ 

I'm aware of the IRS requirements to be considered a nonprofit. However, their considerations for LLC Nonprofits are much more tight and the qualifications are difficult to meet. This is why I am converting to a Corporation.

To answer my question, I'm assuming you're pointing to the idea that I will have to file the business on my personal return as a for profit LLC for this tax year, and then report as the  desired nonprofit status next year after the conversion is done.