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@Ephesians3-14 wrote:

it would require the sole proprietor to get new bank account,

set up a new payroll account with IRS,

notify hundreds of vendors, etc.

The LLC's EIN is ONLY use for payroll and excise taxes.  For income tax purposes it uses the EIN of its owner.

Some banks want the LLC's EIN, but my bank gave me a W-9 which means it was my personal EIN (not the EIN of the LLC).

Payroll stuff, yes.

Vendors?  If it is for W-9s, you still use the EIN of the owner (not the EIN of the LLC).