Plan sponsor change (updated with form 5500 issue)

20 years ago I opened an account with Smith Barney. At the time we filled in me as a sole proprietor and used my social security. Over the years I update the plan with new addresses.  The business of operating under my own name continues to today. During that time I opened a pass through LLC and made contributions via my accountant. At some point I stopped contributions and over the years the account grew to $250k. I paid him to do the first form 5500 which he used my business name and EIN. Thereafter I continued to file using that template. I closed the LLC in Nov.  Since the 401k is in an active business name and SS, I want to keep it open. Form 5500 requires an EIN for which I don't have for the sole prop. Also, the indivdual 401k is in a SS number.  How do you rectify having a new "sponsor" as myself and I assume get an EIN number? 

Only speaking to the question about Sole prop and EIN (not addressing any of your filing or business entity issues). 

Sole proprietors can, and do, get EINs all the time. They are available through the exact same process online as with any other business that needs an EIN, on the IRS website. they might need them for retirement plan purposes, such as here. Or because they have employees and will be doing payroll and remitting payroll taxes and issuing W-2s, etc lots of different reasons why having an EIN might be needed for a sole prop.