If your LLC has or had any W-2 employees in 2019, then you need a new EIN. Look, it only takes 10 minutes (if you type really, really slow) to get a new EIN online at the IRS website I mentioned previously. I would suggest you get a new EIN for the S-Corp, especially if you will be reporting closure of the SCH C business on your 2019 tax return. There is nothing that prohibits you from doing so. It helps the IRS 'keep the books straight". From the IRS perspective you've got a SCH C reporting the closure of the SCH C EIN, and you've got a 1120-S showing the same EIN as opened at the same time the SCH C business was opened. It is invariable that if by chance your 1040 or 1120-S is selected for human review, a human at the IRS will be confused by this and while they will be wrong, *YOU* are the one who has to deal with the headache of the audit. So avoid this possibility and get a new EIN for the S-Corp. Again, there's absolutely nothing that prohibits it.
During the time it took me to type this response, you could already have your new EIN.
I suspect the reason you want to keep the original EIN is because the bank where you have your business account at will not let you just "change the EIN" on the account. They want you to open an entirely new account. But the bottom line is, you're gonna do what you want regardless, since legally you can. It honestly doesn't matter to me one way or the other. It won't change the actions that you have to perform one bit, in order to close the SCH C business and open the S-Corp business.
